Apply suggestions from code review (manually) matrix-org#2

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

synapse/federation/sender/__init__.py

@@ -360,7 +360,7 @@ async def get_federatable_events_and_destinations(
                 ) -> List[Tuple[EventBase, Collection[str]]]:
                     with Measure(self.clock, "get_destinations_for_events"):
                         # Fetch federation destinations per event,
-                        # skip if get_destinations_for_event returns None,
+                        # skip if get_destinations_for_event returns an empty collection,
                         # return list of event->destinations pairs.
                         return [
                             (event, dests)