Actually fix latest-deps mypy errors

Author: David Robertson

synapse/api/filtering.py

@@ -140,13 +140,13 @@
 
 
 @FormatChecker.cls_checks("matrix_room_id")
-def matrix_room_id_validator(room_id_str: str) -> bool:
-    return RoomID.is_valid(room_id_str)
+def matrix_room_id_validator(room_id: object) -> bool:
+    return isinstance(room_id, str) and RoomID.is_valid(room_id)
 
 
 @FormatChecker.cls_checks("matrix_user_id")
-def matrix_user_id_validator(user_id_str: str) -> bool:
-    return UserID.is_valid(user_id_str)
+def matrix_user_id_validator(user_id: object) -> bool:
+    return isinstance(user_id, str) and UserID.is_valid(user_id)
 
 
 class Filtering: