fix: Ensure findById returns null instead of an array when ID is null

Although the phpdocs states that the ID ($id) should not be `null`, in some cases, data may be retrieved from the database where the ID is set to `null`.

When `null` is passed to the `find($id)` method, instead of returning null or a User object, it returns an empty array ([]).

This leads to a type mismatch with the method signature, which expects a return type of `?User`, causing the error:

```
CodeIgniter\Shield\Models\UserModel::findById(): Return value must be of type ?CodeIgniter\Shield\Entities\User, array returned ```
This change ensures that the method always adheres to the expected return type (?User), preventing unexpected runtime errors.

Author: datamweb

src/Models/UserModel.php

@@ -182,7 +182,9 @@ public function fake(Generator &$faker): User
      */
     public function findById($id): ?User
     {
-        return $this->find($id);
+        $result = $this->find($id);
+        
+        return $result instanceof User ? $result : null;
     }
 
     /**