Fix undefined variable on empty Maniphest results pages

Summary: Fixes T2741. If there are no results, `$result_count` is never initialized.

Test Plan: Looked at an empty results page.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2741

Differential Revision: https://secure.phabricator.com/D5328

src/applications/maniphest/controller/ManiphestTaskListController.php

@@ -307,6 +307,7 @@ final class ManiphestTaskListController extends ManiphestController {
           '%s'.
         '</h1>',
         $no_tasks));
+      $result_count = null;
     } else {
       $pager = new AphrontPagerView();
       $pager->setURI($request->getRequestURI(), 'offset');