[FIX] *: avoid cutting 0.0 value in first row

Have in a purchase order a first row with unit price 0
Export the PO adding unit price in the list of fields to export
The unit price is not reported

This occur because the first line of the order lines is meld with the
purchase order line but in the process the 0.0 values is discarded

opw-2510917

closes odoo/odoo#70353

X-original-commit: 242bf502
Signed-off-by: agr-odoo <agr-odoo@users.noreply.github.com>