[FIX] point_of_sale: empty picking

- Create 2 stockable products (A & B).
- Sell these products in the POS, but leave the qty of Product A to
  zero.
- Validate and pay
- In the backend, return the products and register payment

Two pickings are created, and one of them is empty.

Products with a quantity of zero are not included in a picking.
Therefore, we should ignore them when the pickings are created.

Fixes #16292
opw-728101