Allow null for token expiration date

Summary: At least under GitHub, the token value is stored as "null", and not missing. And null > anything is false, so Phabricator thinks the token is expired or not there.

Test Plan: http://ph.vm/settings/panel/external/ before shows "No OAuth Access Token," and after it says "Active OAuth Token".

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7466