R1:68de639470f0

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

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