Fix two edge case issues with Differential mail tags

Summary:
Via Asana. The tags on Differential mail are wrong in two cases:

• Transactions which submit inline comments but no comment text are not labeled as "comments", but should be.
• Non-close, non-comment transactions are not labeled at all, but should be labeled "other".

Test Plan: Submitted a no-comments, inlines-only transaction and got a message with proper X-Phabricator-Mail-Tags header.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7912