Fix empty RewirerMappings.Hydrate being slow

Created by: eseliger

️ Caution! This PR increases performance for searches by up to a bazillion percent, please fasten your seatbelt while reviewing. Thank you for travelling with campaigns.

When there were either no changesets or no changeset specs, the store would return all changesets from the DB, just to forget about them right after. This was really slow for a DB with 15000 changesets, and caused search queries with no results to take roughly 30s. Now we're down to 2-3s.