[FIX] sale: taxes and untaxed amount

- Create a SO, do not set a partner
- Add a product with a tax

The untaxes amount as well as the taxes remain 0.0.

This is because there is no pricelist, therefore no currency.

There is actually no need of rounding explicitly since these are
monetary fields which will be rounded automatically.

opw-1931796

closes odoo/odoo#30450