Fix `newly_left` rooms not appearing if we returned early (Sliding Sync) (#17301)

Fix `newly_left` rooms not appearing if we returned early when `membership_snapshot_token.is_before_or_eq(to_token.room_key)`. 

Introduced in https://github.com/element-hq/synapse/pull/17187 (part of Sliding Sync)

The tests didn't catch it because they had a small typo in it `room_id1` vs `room_id2`.

Found while working on https://github.com/element-hq/synapse/pull/17293