Improve /sync performance of when passing filters with empty arrays. (#14786)

This has two related changes:

* It enables fast-path processing for an empty filter (`[]`) which was
  previously only used for wildcard not-filters (`["*"]`).
* It special cases a `/sync` filter with no-rooms to skip all room
  processing, previously we would partially skip processing, but would
  generally still calculate intermediate values for each room which were
  then unused.

Future changes might consider further optimizations:

* Skip calculating per-room account data when all rooms are filtered (currently
  this is thrown away).
* Make similar improvements to other endpoints which support filters.

synapse/handlers/sync.py

@@ -1403,11 +1403,14 @@ class SyncHandler:
 
         logger.debug("Fetching room data")
 
-        res = await self._generate_sync_entry_for_rooms(
+        (
+            newly_joined_rooms,
+            newly_joined_or_invited_or_knocked_users,
+            newly_left_rooms,
+            newly_left_users,
+        ) = await self._generate_sync_entry_for_rooms(
             sync_result_builder, account_data_by_room
         )
-        newly_joined_rooms, newly_joined_or_invited_or_knocked_users, _, _ = res
-        _, _, newly_left_rooms, newly_left_users = res
 
         block_all_presence_data = (
             since_token is None and sync_config.filter_collection.blocks_all_presence()
@@ -1789,6 +1792,11 @@ class SyncHandler:
             - newly_left_rooms
             - newly_left_users
         """
+
+        # If the request doesn't care about rooms then nothing to do!
+        if sync_result_builder.sync_config.filter_collection.blocks_all_rooms():
+            return set(), set(), set(), set()
+
         since_token = sync_result_builder.since_token
 
         # 1. Start by fetching all ephemeral events in rooms we've joined (if required).