A user with grade assignments capability, but who should not see any students, can see the name of one student

This is related to MDL-56363 and MDL-57362. Steps to reproduce:

1. You need a course with some students in groups, and a non-editing teacher who is in no groups.
2. Create an assignment, set it to separate groups mode.
3. A the non-editing teacher, go to the assignment, and click the Grade button.

Expected result: no students name is shown (matching where it says 0 or 0 students under the user selector).

Actual result: In the middle of the top bar it shows the name and picture of one user.

This is a minor data-protection issue.