When trying to download all assignments for an assignment where there are no submissions, the user sees a completely blank page. I would expect to either:

• see a message explaining that there are no submissions
• receive an empty zip file

To reproduce:

1. Create an assignment (mod_assign) which accepts file submissions
2. Click on "download all submissions"
3. You should see a blank page
4. Make a submission to the assignment
5. Click on "download all submissions" again
6. You should get a zip file containing the submission