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/search.py

@@ -275,7 +275,7 @@ class SearchHandler:
         )
         room_ids = {r.room_id for r in rooms}
 
-        # If doing a subset of all rooms seearch, check if any of the rooms
+        # If doing a subset of all rooms search, check if any of the rooms
         # are from an upgraded room, and search their contents as well
         if search_filter.rooms:
             historical_room_ids: List[str] = []