Fix exception when failing to talk to remote server (#17411)

Broke in #17381

changelog.d/17411.misc

@@ -0,0 +1 @@
+Fix linting errors from new `ruff` version.

synapse/federation/sender/per_destination_queue.py

@@ -322,6 +322,7 @@ class PerDestinationQueue:
         )
 
     async def _transaction_transmission_loop(self) -> None:
+        pending_pdus: List[EventBase] = []
         try:
             self.transmission_loop_running = True
 
@@ -341,7 +342,7 @@ class PerDestinationQueue:
                 self._new_data_to_send = False
 
                 async with _TransactionQueueManager(self) as (
-                    pending_pdus,
+                    pending_pdus,  # noqa: F811
                     pending_edus,
                 ):
                     if not pending_pdus and not pending_edus: