Fixed undefined variable error in catchup (#9664)

Broke in #9640

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

changelog.d/9664.misc

@@ -0,0 +1 @@
+Improve performance of federation catch up by sending events the latest events in the room to the remote, rather than just the last event sent by the local server.

synapse/federation/sender/per_destination_queue.py

@@ -480,6 +480,8 @@ class PerDestinationQueue:
                     # 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