R1:d6dc5d8e68bf

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…