Fixed undefined variable error in catchup

Broke in #9640

synapse/federation/sender/per_destination_queue.py

@@ -480,6 +480,8 @@ async def _catch_up_transmission_loop(self) -> None:
                     # the other sending servers are up).
                     if new_pdus:
                         room_catchup_pdus = new_pdus
+                    else:
+                        room_catchup_pdus[pdu]
 
                 logger.info(
                     "Catching up rooms to %s: %r", self._destination, pdu.room_id