Fix exception when trying to save global setting mail tags

Summary: Ref T11098. Template preferences don't have a user, but this codepath didn't get fully updated to account for that.

Test Plan: Saved mail tags in global prefernces.

Reviewers: avivey, chad

Reviewed By: chad

Maniphest Tasks: T11098

Differential Revision: https://secure.phabricator.com/D16050

src/applications/settings/panel/PhabricatorEmailPreferencesSettingsPanel.php

@@ -160,7 +160,7 @@ final class PhabricatorEmailPreferencesSettingsPanel
     return $editors;
   }
 
-  private function getAllTags(PhabricatorUser $user) {
+  private function getAllTags(PhabricatorUser $user = null) {
     $tags = array();
     foreach ($this->getAllEditorsWithTags($user) as $editor) {
       $tags += $editor->getMailTagsMap();