Minor, don't show references in diffusion if there isn't any

Summary:
When there are no other references (and there will be none ususally,
if D2891 is accepted), we would show "References:" with empty contents.

Avoid that by testing for empty output after applying stupid filtering.

Test Plan: Looked in a standard repository with no special refs.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2892

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -848,6 +848,10 @@ final class DiffusionCommitController extends DiffusionController {
     $refs = explode(',', $refs);
     $refs = array_map('trim', $refs);
 
+    if (!$refs) {
+        return null;
+    }
+
     $ref_links = array();
     foreach ($refs as $ref) {
       $ref_links[] = phutil_render_tag(