[FIX] account: account.move.line calculate non-None values

Functions like `_get_price_total_and_subtotal` shoukd be able to handel falsy values like 0 or empty recordset,
currently invoking them with this values (e.g. discount=0) leads to use record values instead of given values.

closes odoo/odoo#91005

Signed-off-by: William André (wan) <wan@odoo.com>