Fix two Pholio subscriptions issues

Summary:
  - Grouping inline comments made us render `array(null)` for the content of a single comment. Detect that this corresponds to empty text.
  - Don't try to mention-subscribe automatic subscribers, like the author.

Test Plan: Tried to mention-subscribe the author, saw no "x subscribed y" transaction generate. Added a real subscriber, saw no empty content render.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5315