Fix the "x" link in tokenizer tokens incorrectly closing dialogs

Summary:
See https://discourse.phabricator-community.org/t/removing-tags-or-subscribers-from-a-maniphest-modal-causes-the-modal-to-close/2874.

My Javascript is rusty: '' + null == 'null'. Same for undefined. Use an explicit typecheck instead.

Test Plan: Clicked the "x" in a tokenizer token in a dialog, saw the token removed instead of the dialog closed.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20623