Minor, really fix empty reference properties in diffusion

Summary:
Oh man, after the explode/map, the output when no references is actually

array(1) {
  [0]=> string(0) ""
}

.. making the falsey check fail to work as expected.

Test Plan: same as D2892, but with a little more scrutiny :p

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2893