Only check change cache if we know stream ordering

We have receipts without valid events in our database which is supported
by synapse for unknown historical reasons (I know).

Author: Fizzadar

synapse/storage/databases/main/event_push_actions.py

@@ -793,7 +793,7 @@ def _get_notif_unread_count_for_user_room(
 
         # If there have been no events in the room since the stream ordering,
         # there can't be any push actions either.
-        if not self._events_stream_cache.has_entity_changed(
+        if stream_ordering and not self._events_stream_cache.has_entity_changed(
             room_id, stream_ordering, max_stream_ordering
         ):
             return []