Fix phpstan issues

Author: magicsunday

src/JsonMapper/Type/TypeResolver.php

@@ -62,7 +62,7 @@ public function resolve(string $className, string $propertyName): Type
 
         $type = $this->extractor->getType($className, $propertyName);
 
-        if ($type === null) {
+        if (!($type instanceof Type)) {
             $type = $this->resolveFromReflection($className, $propertyName);
         }
 

tests/JsonMapper/Type/TypeResolverTest.php

@@ -249,7 +249,7 @@ public function getType(string $class, string $property, array $context = []): ?
     /**
      * @param array<array-key, mixed> $context
      */
-    public function getTypes(string $class, string $property, array $context = []): ?array
+    public function getTypes(string $class, string $property, array $context = []): null
     {
         return null;
     }