Fix HTTP 400 / 500 errors in Pholio

Summary:
Ref T2650. Possible fix for that issue.

  - "Passthru" got renamed to "passthrough" but a site was missed.
  - Don't try to post an empty comment if the text is empty but we have inlines; this avoids popping a "you can't post an empty comment" error.

Test Plan: Made an empty comment with an inline in Pholio.

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2650

Differential Revision: https://secure.phabricator.com/D5240