Fix handling of implicit comment transaction in paste creation

Summary:
Fixes T9850. The getComment() test should be a hasComment() test, in order to discard empty comments.

Also backport a couple of future fixes which can get you into trouble if you reconfigure forms in awkward ways.

Test Plan: Created a new paste without a comment.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9850

Differential Revision: https://secure.phabricator.com/D14571