Fixes an issue where updates to the first post in a topic would be visible only for staff.
Before, because the empty tag would find its way to `PostRevisor`, `TopicsController#update` would create a hidden revision, and later `PostsController#update` would only update that revision.
After this change, `TopicsController` doesn't create a revision at all (unless necessary), so `PostsController` can create it (and correctly decide whether the revision should be hidden or not).
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