Fix overzealous subscribing in EditEngine

Summary:
See T9905#148799. The CommentEditField generated empty comment transactions; these are dropped later, but before they are dropped they would trigger implicit CCs.

The implicit CC rule should probably be narrower, but we shouldn't be generating these transactions in the first place.

Test Plan: No longer implicitly CC'd on a task when doing something minor like changing projects.

Reviewers: chad

Reviewed By: chad

Subscribers: avivey

Differential Revision: https://secure.phabricator.com/D14795