Actually return something sensible

synapse/storage/event_push_actions.py

@@ -72,7 +72,7 @@ def _get_unread_event_push_actions_by_room(txn):
             )
             results = txn.fetchall()
             if len(results) == 0:
-                return {}
+                return {"notify_count": 0, "highlight_count": 0}
 
             stream_ordering = results[0][0]
             topological_ordering = results[0][1]