Sliding Sync: Short-circuit have_finished_sliding_sync_background_jobs (#17723)

We only need to check it if returned bump stamp is `None`, which is rare.

Pulling this change out from one of @erikjohnston's branches
(https://github.com/element-hq/synapse/compare/develop...erikj/ss_perf)

changelog.d/17723.misc

@@ -0,0 +1 @@
+Fetch `bump_stamp`'s more efficiently in Sliding Sync.

synapse/handlers/sliding_sync/__init__.py

@@ -1171,8 +1171,8 @@ class SlidingSyncHandler:
             # `SCHEMA_COMPAT_VERSION` and run the foreground update for
             # `sliding_sync_joined_rooms`/`sliding_sync_membership_snapshots`
             # (tracked by https://github.com/element-hq/synapse/issues/17623)
-            await self.store.have_finished_sliding_sync_background_jobs()
-            and latest_room_bump_stamp is None
+            latest_room_bump_stamp is None
+            and await self.store.have_finished_sliding_sync_background_jobs()
         ):
             return None