fix: include 'extra_fields' conditionally

Author: diegocastrum

django_celery_results/managers.py

@@ -161,8 +161,11 @@ def store_result(self, content_type, content_encoding,
             'task_args': task_args,
             'task_kwargs': task_kwargs,
             'worker': worker,
-            'extra_fields': extra_fields
         }
+
+        if extra_fields is not None:
+            fields.update(extra_fields)
+
         if 'date_started' in kwargs:
             fields['date_started'] = kwargs['date_started']