Select to_key if to_key else from_key

See #17320 (comment)

synapse/storage/databases/main/stream.py

@@ -1827,15 +1827,19 @@ async def paginate_room_events(
             and to_key is not None
             and to_key.is_before_or_eq(from_key)
         ):
-            return [], from_key
+            # Token selection matches what we do in `_paginate_room_events_txn` if there
+            # are no rows
+            return [], to_key if to_key else from_key
         # Or vice-versa, if we're looking backwards and our `from_key` is already before
         # our `to_key`.
         elif (
             direction == Direction.BACKWARDS
             and to_key is not None
             and from_key.is_before_or_eq(to_key)
         ):
-            return [], from_key
+            # Token selection matches what we do in `_paginate_room_events_txn` if there
+            # are no rows
+            return [], to_key if to_key else from_key
 
         rows, token = await self.db_pool.runInteraction(
             "paginate_room_events",