Fix global search scope fatal on 404 page (or other pages with no Application)

Summary: Ref T13405. Some pages don't have a contextual application.

Test Plan: Viewed 404 page, no more fatal.

Maniphest Tasks: T13405

Differential Revision: https://secure.phabricator.com/D20792

src/view/page/menu/PhabricatorMainMenuSearchView.php

@@ -118,7 +118,7 @@ final class PhabricatorMainMenuSearchView extends AphrontView {
 
   public static function getGlobalSearchScopeItems(
     PhabricatorUser $viewer,
-    PhabricatorApplication $application) {
+    PhabricatorApplication $application = null) {
 
     $items = array();
     $items[] = array(