bugfix: ensure we can sync with SSS even with missing rooms

Fixes element-hq/element-x-ios#3300

Author: kegsay

synapse/storage/databases/main/stream.py

@@ -1469,6 +1469,10 @@ async def bulk_get_last_event_pos_in_room_before_stream_ordering(
         recheck_rooms: Set[str] = set()
         min_token = end_token.stream
         for room_id, stream in uncapped_results.items():
+            if stream is None:
+                # Despite the function not directly setting None, the cache can!
+                # See: https://github.com/element-hq/synapse/issues/17726
+                continue
             if stream <= min_token:
                 results[room_id] = stream
             else: