I see! So we need a way to differentiate between "Don't do anything" and "Change grader type to ungraded". It seems to me what makes the most amount of sense is to set `grader_type` to `False` if the section is meant to be ungraded, and continue to use `None` when we don't make any change to whether or not the sequence is graded.

Also, in this case, we don't even really need to mention `GRADER_NOT_SUPPLIED` since item.py won't `CourseGradingModel.update_section_grader_type` if `grader_type == GRADER_NOT_SUPPLIED`