Fix multiple grading to grade empty as empty

Summary:
This was a bad one. :(

I'm not sure what this .attr("required") was supposed to be... do we have cases where answers are optional (besides fallbacks)?

Test Plan: Ran answer-types unit tests.

Reviewers: eater, emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D2922