fix(metadata): reader should be nullable (api-platform#5378)

Author: louismariegaborit

src/Core/Metadata/Property/Factory/AnnotationSubresourceMetadataFactory.php

@@ -83,6 +83,10 @@ public function create(string $resourceClass, string $property, array $options =
                 return $this->updateMetadata($attributes[0]->newInstance(), $propertyMetadata, $resourceClass, $property);
             }
 
+            if (!$this->reader) {
+                return $propertyMetadata;
+            }
+
             $annotation = $this->reader->getMethodAnnotation($reflectionMethod, ApiSubresource::class);
             if ($annotation instanceof ApiSubresource) {
                 return $this->updateMetadata($annotation, $propertyMetadata, $resourceClass, $property);