fix Possibly invalid array key type string|null.

Author: roberto-butti

src/Block.php

@@ -165,8 +165,12 @@ public function set(int|string $key, mixed $value, string $charNestedKey = "."):
 
                 $array = &$array[$key];
             }
+            $key = array_shift($keys);
+
+            if (!is_null($key)) {
+                $array[$key] = $value;
+            }
 
-            $array[array_shift($keys)] = $value;
             return $this;
         }
         $this->data[$key] = $value;

tests/Feature/QueryBlockTest.php

@@ -134,7 +134,6 @@
 
     $story = Block::fromJsonString($jsonString);
     $assets = $story->extractWhere("fieldtype", "asset");
-    $assets->dump();
     expect($assets)->toHaveCount(16);
     expect($assets->get("3.filename"))->toStartWith("https://a.story");
 });