[FIX] sale: calculate untaxed_amount_to_invoice based on invoiced quantity

Enable discounts on sales order lines
Create a service type product with invoicing based on timesheet and
task in sales order's project
Create so for 6 hours and deliver 8 hours.
Create 2 invoices: One for 6 hours with the regular price and 2 hours
h 100% discount.
Invoice status of the order will be Fully Invoiced

Go to Sales>Reporting>Sales Analysis report will show the untaxed amount
associated with 2nd invoice as still to invoice which is incorrect

This occur because when saving the invoice the untaxed amount is
calculated on the amount invoice after discount

opw-2338564

closes odoo/odoo#58908

Signed-off-by: agr-odoo <agr-odoo@users.noreply.github.com>