Metadata: Attributes default value should be null not an empty array

Author: soyuka

src/Metadata/Resource/Factory/AnnotationResourceMetadataFactory.php

@@ -85,7 +85,7 @@ private function createMetadata(ApiResource $annotation, ResourceMetadata $paren
                 $annotation->iri,
                 $annotation->itemOperations,
                 $annotation->collectionOperations,
-                $annotation->attributes,
+                $annotation->attributes ?: null,
                 $annotation->subresourceOperations,
                 $annotation->graphql
             );