Manual grading page is broken when Question per page is set incorrectly.

Steps to reproduce:

1. Create a course.
2. Create a quiz activity.
3. Add an "Essay" question.
4. Login as a student, and submit the quiz.
5. Login as teacher,  go to Manual grading screen.
6. Press "grade" link to make a grade.
7. Set 0 in "Questions per page" in "Options", and press "Change options".
8. The error "Coding error detected, it must be fixed by a programmer: moodle_database::get_in_or_equal() does not accept empty arrays" is shown, and the user cannot manual grade any more.