fix(phpstan): apply phpstan fixes (#2721)

## Description

Fixes failing PHPStan

```                                                                                                                                                                                                                                                                                                                       
 ------ ------------------------------------------------------------------                                                                                                                                                                                                                                                
  Line   src/Model/ModelRegistry.php                                                                                                                                                                                                                                                                                      
 ------ ------------------------------------------------------------------                                                                                                                                                                                                                                                
  122    Variable $schemaJson on left side of ?? always exists and is not                                                                                                                                                                                                                                                 
         nullable.                                                                                                                                                                                                                                                                                                        
         🪪  nullCoalesce.variable                                                                                                                                                                                                                                                                                        
 ------ ------------------------------------------------------------------                                                                                                                                                                                                                                                
                                                                                                                                                                                                                                                                                                                          
 ------ -----------------------------------------------------------------------                                                                                                                                                                                                                                           
  Line   src/Util/LegacyTypeConverter.php                                                                                                                                                                                                                                                                                 
 ------ -----------------------------------------------------------------------                                                                                                                                                                                                                                           
  50     Strict comparison using === between 'null' and                                                                                                                                                                                                                                                                   
         'array'|'int'|'object'|'string' will always evaluate to false.                                                                                                                                                                                                                                                   
         🪪  identical.alwaysFalse                                                                                                                                                                                                                                                                                        
         💡  If Symfony\Component\PropertyInfo\Type::isCollection() is impure,                                                                                                                                                                                                                                            
         add @phpstan-impure PHPDoc tag above its declaration. Learn more: htt                                                                                                                                                                                                                                            
         ps://phpstan.org/blog/remembering-and-forgetting-returned-values                                                                                                                                                                                                                                                 
         💡  If Symfony\Component\PropertyInfo\Type::getClassName() is impure,                                                                                                                                                                                                                                            
         add @phpstan-impure PHPDoc tag above its declaration. Learn more: htt                                                                                                                                                                                                                                            
         ps://phpstan.org/blog/remembering-and-forgetting-returned-values                                                                                                                                                                                                                                                 
         💡  If Symfony\Component\PropertyInfo\Type::getCollectionValueTypes()                                                                                                                                                                                                                                            
         is impure, add @phpstan-impure PHPDoc tag above its declaration. Lear                                                                                                                                                                                                                                            
         n more: https://phpstan.org/blog/remembering-and-forgetting-returned-                                                                                                                                                                                                                                            
         values                                                                                                                                                                                                                                                                                                           
         💡  If Symfony\Component\PropertyInfo\Type::getCollectionKeyTypes() is                                                                                                                                                                                                                                           
         impure, add @phpstan-impure PHPDoc tag above its declaration. Learn m                                                                                                                                                                                                                                            
         ore: https://phpstan.org/blog/remembering-and-forgetting-returned-val                                                                                                                                                                                                                                            
         ues                                                                                                                                                                                                                                                                                                              
 ------ -----------------------------------------------------------------------                                                                                                                                                                                                                                           
```

## What type of PR is this? (check all applicable)
- [x] Bug Fix
- [ ] Feature
- [ ] Refactor
- [ ] Deprecation
- [ ] Breaking Change
- [ ] Documentation Update
- [x] CI

## Checklist
- [ ] I have made corresponding changes to the documentation (`docs/`)

Author: DjordyKoert

src/Model/ModelRegistry.php

@@ -119,7 +119,7 @@ public function register(Model $model): string
             $this->unregistered = array_unique($this->unregistered);
             // Only store schema if it was successfully generated
             if (null !== $schema) {
-                $this->schemaToModelMap[$identifier][$schemaJson ?? json_encode($schema)] = $model;
+                $this->schemaToModelMap[$identifier][$schemaJson] = $model;
             }
         }
 

src/Util/LegacyTypeConverter.php

@@ -47,7 +47,7 @@ public static function toTypeInfoType(?array $legacyTypes): ?Type
                 default => throw new \LogicException('Unsupported LegacyType type: '.$legacyType->getBuiltinType().'.'),
             };
 
-            if (LegacyType::BUILTIN_TYPE_NULL === $legacyType->getBuiltinType() || $legacyType->isNullable()) {
+            if ($legacyType->isNullable()) {
                 $nullable = true;
             }
         }