Don't provide Undo for empty text in Differential inline comment

Test Plan:
Edit inline comment.
Clear its text.
Hit Cancel.
Try to find Undo link on the usual place with no success.
Repeat for same text.
Repeat for different text, this time with success.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1739