Handle widgets which only exist in one revision

Summary:
This would cause a Javascript error on some items, the problem was that
if there was only one widget, it would be considered text, so its
`value` would be undefined, causing an error.

Test Plan:
* In difftest.html, add a new widget to exactly one of the items.
* Should not get an error

Reviewers: alex

Reviewed By: alex

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7829