Only show submissions without course in the submission history of a submission without a course

[jorg-vr]

Closes #3544

[chvp]

How does this interact with a page like https://dodona.ugent.be/nl/activities/1388161683/submissions/? When we re-use previous exercises for examinations/evaluations we use pages like that to check that students in the current course did not submit to a certain exercise. They could have done that both inside and outside of a course, so it is important that we can see both types of submissions.

[jorg-vr]

This code does not change anything to that page.

The change in app/controllers/submissions_controller.rb only affects submission show

The change in app/models/submission.rb makes the in_course filter support nil values
I highly doubt that this page does any filtering on course, and other pages that do use the filter already should not have provided nil values before as these would have resulted in errors.

Thus no behavior other than on the submission show page