Fix "Resign as Reviewer" to appear only when actually a Reviewer

Summary: Is this an appropriate place to say "herp derp?"

Test Plan: Check for Resign as Reviewer for the following table
                          waiting for review | accepted | committed
Am actually reviewer
Am not reviewer but CC'ed
Unrelated

Reviewers: epriestley

CC:

Differential Revision: 140

src/applications/differential/controller/revisionview/DifferentialRevisionViewController.php

@@ -506,7 +506,7 @@ class DifferentialRevisionViewController extends DifferentialController {
 
     $actions[DifferentialAction::ACTION_ADDREVIEWERS] = true;
 
-    return array_keys($actions);
+    return array_keys(array_filter($actions));
   }
 
   private function loadInlineComments(array $comments, array &$changesets) {