[FIX] purchase: seller selection

- Create an article with one supplierinfo (with a delivery delay and a
  min. quantity > 0) and a customer delivery delay
- Trigger the creation of a PO by a procurement order for this article

The supplier delivery delay is not taken into account in the date order
of the PO because the seller is not selected.

This is because the procurement quantity is not sent to the seller
selection.

opw-16702
Closes #16702