Make use of the mypy plugin change

as spotted here
#12717 (comment)

Author: David Robertson

synapse/storage/databases/main/roommember.py

@@ -15,6 +15,7 @@
 import logging
 from typing import (
     TYPE_CHECKING,
+    Callable,
     Collection,
     Dict,
     FrozenSet,
@@ -634,7 +635,7 @@ def _get_users_server_still_shares_room_with_txn(
         )
 
     async def get_rooms_for_user(
-        self, user_id: str, on_invalidate=None
+        self, user_id: str, on_invalidate: Optional[Callable[[], None]] = None
     ) -> FrozenSet[str]:
         """Returns a set of room_ids the user is currently joined to.