Fix Jump to HEAD link

Test Plan:
Jump to head.
Go to doctor.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2097

src/applications/diffusion/controller/base/DiffusionController.php

@@ -292,7 +292,7 @@ abstract class DiffusionController extends PhabricatorController {
         array(
           'href' => $drequest->generateURI(
             array(
-              'commit' => null,
+              'commit' => '',
             ) + $uri_params),
         ),
         'Jump to HEAD');