add a key existence check to tags_by_room to avoid /events 500'ing when testing against vector

synapse/storage/tags.py

@@ -95,6 +95,7 @@ class TagsStore(SQLBaseStore):
         if room_ids:
             tags_by_room = yield self.get_tags_for_user(user_id)
             for room_id in room_ids:
+                if room_id in tags_by_room:
                     results[room_id] = tags_by_room[room_id]
 
         defer.returnValue(results)