Assignment invitation: If submission_content is empty, remove the existing withContent field from the head

[enrubio]

For venues using tracks - if Paper Matching Setup is first run with a submission track selected it will update the withContent param of the head in the Assignment invitation. But if they later run it for all papers (submission_track is empty) the assignment invitation is not updated.

Maybe in set_assignment_invitation we should check if submission_content is empty and update the assignment invitation.