[FIX] point_of_sale: pos order to invoice wrong value

When a pos order is sent to the server, the field 'to_invoice' is never
sent to the server which lead to always having a value to 'false'.

We are now sending correctly this value to the server, and we can also
base the fact to create an invoice on the value of each order.

Because beeing offline could lead to create an order that should be
invoiced as one that should not be.

closes odoo/odoo#39843

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