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.
