Grade view crashes when any submission is unidentified
In https://gitlab.kwant-project.org/zesje/zesje/-/commit/15b280f12b8b31ae40e808490cc47d7c16b810a3#cc1c3752fde923be0ac00a3cf78377fa3cac8de4_535_534 `sub.student` can be null, which causes a crash when `sub.student.id` is accessed.
issue