Fix an issue with mentioning revisions on the new EditEngine code

Summary:
Ref T12020. Ref T11114. If we continue here on a mention, we try to generate $old, which requires reviewers to be attached. They won't be for simple codepaths like mentions.

Instead, just bail early: we don't need to do anything anyway since we can't possibly find any more errors with zero transactions.

Test Plan: Mentioned a revision on a task.

Reviewers: chad, avivey

Reviewed By: avivey

Maniphest Tasks: T11114, T12020

Differential Revision: https://secure.phabricator.com/D17059