Don't error if a question doesn't have `widgets`

Summary:
I did not realize:

  _.keys(undefined) // => error

This commit fixes the call to _.keys to account for values that might
not be present

Test Plan:
* Load up difftest.html with a diff missing the widgets object in one or both
  of the items
* See no error

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7267