Make marking comments as "Done" work cleanly on EditEngine

Summary: Ref T11114. Fixes T10323.

Test Plan:

• Marked comments as done only: no warning about not leaving a comment.
• Did nothing: warning about posting an empty comment.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114, T10323

Differential Revision: https://secure.phabricator.com/D17120