Don't accept unanswered questions for basic_set_notation

Test plan:
In local mode,
- Leave the answer area blank and submit. Answer shouldn't be accepted.
- Enter something in the text box and submit. Answer should be marked wrong.
- Check the checkbox and leave the text box blank. Answer should be marked wrong.
- Enter something in the text box AND check the checkbox. Answer shouldn't be accepted.
- Enter the correct answer. Answer should be marked correct.

Auditors: cbhl