Sliding Sync: Short-circuit have_finished_sliding_sync_background_jobs when checking bump_stamp

[MadLittleMods]

Short-circuit have_finished_sliding_sync_background_jobs when checking bump_stamp. 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 (develop...erikj/ss_perf)

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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  (run the linters)

[devonh]

Nice little improvement

[MadLittleMods]

Thanks for the review @devonh 🐬