In case of no assignment submissions, download-best-solutions produces empty (0B) file instead of proper empty zip file #536

If there are no best solutions of an assignment, exercise-assignments/{id}/download-best-solutions API call (or clicking the Download bests (or invoking) produces an invalid ZIP file

It would be better to return valid (but empty) ZIP file instead

Base64 encoding of an empty zip file is UEsFBgAAAAAAAAAAAAAAAAAAAAAAAA==