Fix a harmless exception when the staged events queue is empty. (#10592)

changelog.d/10592.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in v1.37.1 where an error could occur in the asyncronous processing of PDUs when the queue was empty.

synapse/federation/federation_server.py

@@ -972,11 +972,16 @@ class FederationServer(FederationBase):
         # the room, so instead of pulling the event out of the DB and parsing
         # the event we just pull out the next event ID and check if that matches.
         if latest_event is not None and latest_origin is not None:
-            (
-                next_origin,
-                next_event_id,
-            ) = await self.store.get_next_staged_event_id_for_room(room_id)
-            if next_origin != latest_origin or next_event_id != latest_event.event_id:
+            result = await self.store.get_next_staged_event_id_for_room(room_id)
+            if result is None:
+                latest_origin = None
+                latest_event = None
+            else:
+                next_origin, next_event_id = result
+                if (
+                    next_origin != latest_origin
+                    or next_event_id != latest_event.event_id
+                ):
                     latest_origin = None
                     latest_event = None