(fix): rewritten_by was not being cleared in stats (#110)

fix: Don't attempt to get rewrite document if it is null

server/analytics/stats/featuremedia_updates.py

@@ -225,7 +225,7 @@ def finish(self, sender):
         }
 
         def get_parent_id(doc):
-            if 'rewrite_of' not in doc:
+            if not doc.get('rewrite_of'):
                 return doc[config.ID_FIELD]
             elif doc['rewrite_of'] not in docs:
                 # If the parent item was not part of this stats iteration