Resource related fields can return blank.

It can happen when `rest_framework.fields.SkipField` is called.

Author: stas

rest_framework_json_api/renderers.py

@@ -375,6 +375,9 @@ def extract_included(cls, fields, resource, resource_instance, included_resource
                 serializer_data = field.data
 
             if isinstance(field, relations.RelatedField):
+                if not serializer_data:
+                    continue
+
                 if relation_instance is None:
                     continue