Possibly fix issue where refs engine finds empty refs?

Summary:
@mbishopim3 reported an issue in IRC:

> mbishopim3: epriestley: "Error updating working copy: Commit "" has not been discovered yet! Run discovery before updating refs." any ideas?

I can't reproduce it and it went away for him, but one theory is that we're getting here and git/hg are spitting out nothing, which we incorrectly parse as `array("")` when we intend `array()`.

Test Plan:
Pushed some new commits, ran `bin/repositoy refs X`, got expected results.

I can't actually reproduce the bug, but this might fix it and appears to make the code more correct.

Reviewers: btrahan

Reviewed By: btrahan

CC: mbishopim3, aran

Differential Revision: https://secure.phabricator.com/D8326