get_item_list does not [item qty=0]

- By using `if not d.qty:` it totally limits the chance of creating an empty Delivery Note 
- Should be changed to `if d.qty is None` 

Use case for empty DN: 
- Open a Delivery Note and tight to a specific Sales Order first without knowing the real qty at this point.
- Using barcode scanner to receive Sales Order to this DN (multiple times).
Merge pull request #2140 from nathando/patch-1

get_item_list does not [item qty=0]