Catch undefinied title attribute in lightbox

Summary: If the image name in not defined, don't set it. Fixes T6793

Test Plan: View a lightboxed image, no 'undefined' alt tag

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6793

Differential Revision: https://secure.phabricator.com/D11037