Steps to reproduce the bug:

- Let's consider a delivery carrier DC with invoice policy = 'real'
- Let's consider a consumable product P with a weight = 1kg and sales price = 10€
- Create a sale order SO with 2 P and add DC as shipping cost
- Process the shipment for 1 P and create a backorder
- Process the second shipment with the last P

Bug:

Two lines L1, L2 with DC were created on SO but only L1 as a price unit and a description.
L2 had a price unit = 0€ and no description.

opw:2520135

closes odoo/odoo#70454

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>
Co-authored-by: simongoffin <sig@odoo.com>