Require purchase price only from confirm

We may not know the price of the product when filling a purchase. This is often the purpose of the quotation state. I think we should allow empty unit price until confirmation. So user can sent a request for quotation without prices and when he receives the quotation, he can edit the purchase in draft to set the prices and then confirm it.

This will also improve the workflow with #12494 (closed) as purchase price may be empty.