Transformer: Add transformer-only option to grade if empty

Summary:
Adds a checkbox and prop to the transformer widget only to allow grading
if it is empty.

Test Plan:
 * Make a transformer with a multiple choice box answer.
 * Try grading with and without the checkbox set.
 * Make sure a non-answered transformer prevents grading if the
   checkbox is unchecked, but does not prevent grading if the checkbox
   is checked.

Reviewers: alex

Reviewed By: alex

CC: alpert, eater

Differential Revision: http://phabricator.khanacademy.org/D5580