Apply suggestions from code review

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

changelog.d/14917.misc

@@ -1 +1 @@
-Improve performance of looking up partial-state status.
+Faster joins: Improve performance of looking up partial-state status of rooms.

synapse/handlers/sync.py

@@ -1408,7 +1408,7 @@ class SyncHandler:
             forced_newly_joined_room_ids.update(
                 room_id
                 for room_id, is_partial_state in results.items()
-                if is_partial_state
+                if not is_partial_state
             )
 
         # Now we have our list of joined room IDs, exclude as configured and freeze