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