use component's reference for serializers without model

Author: gnuletik

rest_framework/schemas/openapi.py

@@ -517,8 +517,8 @@ def _get_request_body(self, path, method):
 
         if not isinstance(serializer, serializers.Serializer):
             item_schema = {}
-        elif hasattr(serializer, 'Meta') and hasattr(serializer.Meta, 'model'):
-            # If the serializer uses a model, we should use a reference
+        elif hasattr(serializer, 'Meta'):
+            # If possible, the serializer should use a reference
             item_schema = self._get_reference(serializer)
         else:
             # There is no model, we'll map the serializer's fields