[FIX] point_of_sale: picking creation after payment

Since the below revision:
32b5515a

It was no longer possible to create a payment
for a pos order without picking.

Indeed, if `picking_type` is not set,
`picking_id`, which is initialized to `False`,
remains as `False`, and you therefore can't
ask it's `.id` (`picking_id.id`).

opw-687597