Fix query performance for /sync

Broke in #12729.

Author: erikjohnston

synapse/storage/databases/main/stream.py

@@ -750,7 +750,7 @@ def _f(txn: LoggingTransaction) -> Optional[Tuple[int, int, str]]:
                 WHERE room_id = ?
                     AND stream_ordering <= ?
                     AND NOT outlier
-                    AND rejections.reason IS NULL
+                    AND rejections.event_id IS NULL
                 ORDER BY stream_ordering DESC
                 LIMIT 1
             """