Minor fixes for Basic Set Notation

Summary:
- Handle the case when the empty set is the correct answer.
- Clarify problem text.
- Update hints.
- Various coding style nits.

Test Plan: locally, Chrome/OS X

Reviewers: eater, stephanie, alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D2031