Removed data-ensure clauses

Fixes performance issues.
Allow empty answers to be correct when they pass

handleSubmit() didn't entertain the idea that empty answers can still
be correct. I've tried all the other exercises with 'custom' answers
and can't find anything that breaks.

I could just add an 'empty set' checkbox ala
absolute_value_equations.html or button that adds a empty sign to the
field but it seems unintuitive.
Minor hint changes