[FIX] delivery: Backorder shipping cost with invoice policy in real

port of this fix
https://github.com/odoo/odoo/pull/70454



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-3219711

closes odoo/odoo#125379

X-original-commit: 4ce4f9444e5050ae30e4468ff24aecb573045dc7
Signed-off-by: Adrien Widart (awt) <awt@odoo.com>
Signed-off-by: Walid Hanniche (waha) <waha@odoo.com>