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

CCBUG: 484665
CCBUG: 484972