timemodified for grades can be set even when the grade is still null. This leads to circumstances where teachers are misinformed as to which grades are still pending a grade.

Steps to reproduce:

1. In a course, create an assignment.
   1. Under submission types allow for 'Online text'
2. Login as Student
3. Submit anything for your assignment.
4. Login as Teacher/Admin
5. Click on that assignment.
6. You should see: 1 for submitted and 1 for needs grading.
7. Click on View/grade all submissions.
8. Edit grade.
9. Immediately save without changing the grade or feedback.
10. Head back to the assignment page.
11. You should see: Shows 1 for submitted and 0 for needs grading.

You'd assume if the grade is still NULL, despite adding feedback or not that it would indicate the grade has not been graded.
This is consistent with what the student sees, as it indicates on their view that their Grading status is Not graded.

Changing this should not effect the behaviour of notifying a student when feedback is submitted, regardless of if the grade is graded.