We had a bug with setting empty captions for file attachments. After setting

an empty caption, the published caption (if it exists) would be displayed

instead of the empty caption. This was due to the conditions that we used to

serialize captions for draft file attachments. This change fixes that up, and

adds unit tests for this case. While this was a backend problem, I've also

added a JS test for saving an empty caption just for good measure.

Tested setting empty captions on new drafts, new revisions, and published

  file attachments.
Ran unit and JS tests.