Fix an issue where inline comments with only edit suggestions are considered empty

Summary:
Ref T13513. An inline is not considered empty if it has a suggestion, but some of the shared transaction code doesn't test for this properly.

Update the shared transaction code to be aware that application comments may have more complex emptiness rules.

Test Plan:

• Posted an inline with only an edit suggestion, comment went through.
• Tried to post a normal empty comment, got an appropriate warning.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21287