[FIX] purchase_stock: vendor code

- Create a product P with a MTO Reordering Rule (Buy Route)
- Add a supplier to P with a specific vendor code
- Create a SO for 1 unit of P, validate

A PO is created for the supplier but without using the vendor code.

It happens because the vendor code is overridden by the product
description.

To avoid losing information and redundancy, we add the line description
only if different from the product name.

opw-2383418

closes odoo/odoo#61922

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>