Hello Jason,
Why would you want to save a shopping carts without items ? A shopping cart without items has no specific data that needs to be saved. In the default settings you can pre-set data which will apply to all items in the shopping cart and that data doesn't need to be saved.
Also an error will be generated when saving a shopping cart without entering any line items.When the shopping carts has line items the shopping cart can be saved and will get status Hold.
Hope this helps :-)
Antoinette