Fix rendering of comments deleted by editing

Summary:
Fixes T4609. Steps are:

• Make a comment.
• Edit it.
• Delete all the text.

We expect to see "This comment has been deleted." -- instead, things currently render goofy.

Root cause is that hasComment() means both "comment object exists" and "comment object is nonempty".

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4609

Differential Revision: https://secure.phabricator.com/D8533