Diffusion - fix T3215

Summary: Ref T2784. We need to always return array() here so the foreach doesn't crap out

Test Plan: chad - can you confirm this fixes the error? I have ref-heavy git repos it seems. should work though.

Reviewers: chad, epriestley

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T3215, T2784

Differential Revision: https://secure.phabricator.com/D5961

src/applications/diffusion/conduit/ConduitAPI_diffusion_refsquery_Method.php

@@ -35,7 +35,7 @@ final class ConduitAPI_diffusion_refsquery_Method
     // similar to (remote/one, remote/two, remote/three)
     $refs = trim($stdout, "() \n");
     if (!$refs) {
-      return null;
+      return array();
     }
     $refs = explode(',', $refs);
     $refs = array_map('trim', $refs);