[FIX] point_of_sale: create invoice when offline

When a pos order is created and should be invoiced, the argument
to_invoiced is sent when the order is finalized. But if you were offline
during this sale, when you are back online, the 'to_invoice' is lost,
because the value stored on the order is not sent to the server.

So we are now sending the value from the order.

closes odoo/odoo#39884

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