Handle orders that are already partially returned

This change allows for automatic processing even if an order is already
partially returned. If, for example, the payload indicates that we
should create a customer return for items a, b and c but item a was
already returned, this change allows for automatic processing of b and
c. Previously, this scenario would have resulted in a Wombat error.
this is probably my code from earlier, but im having trouble wrapping my head around why we need to check for non-empty items from payload.