Don't allow enqueueing all items in a view if the view is empty
Prevents the user from triggering a crash when enqueueing in an empty Files view.
Test-plan:
- Set Elisa to index an empty directory, go into albums view, try to enqueue all items
- Use the Files view to navigate into an empty directory, try to enqueue all items
Before: crash when enqueuing from files, nothing happens when enqueuing from albums view
Now: buttons to enqueue are disabled