Step Builder: Fix JS error that causes crash if no review step is present

[itsjeyd]

No description provided.

[bradenmacdonald]

👍 Works for me.

If you have time though, I think this could be cleaned up a bit more: gradeDOM could be owned by the review step JS rather than the step builder JS. Also, gradeDOM does not need to be a DOM element at all, since it is invisible. It's only used as a hack for passing grade data to the JS. A better way would be to pass the grade data as the second argument to fragment.initialize_js('MentoringWithStepsBlock', data={'grade': self.get_grade()}) and then store the data in a JavaScript data structure owned by the review step JS code.