Correctly handle downloading ZIP with empty directories in Phragment
Summary: This logic causes an exception because getPHID() is called on a fragment that has no latest version. This fixes the code so that in this scenario, it returns an empty array (with no path to be added to the ZIP).
Test Plan: Downloaded the ZIP successfully after the patch was applied.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T4205
Differential Revision: https://secure.phabricator.com/D7735