CourseKeyField serializes None as empty string; however, when we searched for `course_id=None`, we got errors and we can't search for `course_id=''` as empty string is not a CourseKey. (org works since it sets the column to NULL if it's None). I guess I could fetch all the records and filter them in python before calling delete? I should at least add a test asserting both an OrgRole and a CourseRole in that org for the same user and then retracting the OrgRole.