[FIX] payment: fallback on transaction partner parent name

In a Sale Order, it is possible to select an invoice address without a
name. The partner of the transaction does not have a name, which leads
to a traceback when trying to split the name.

The idea is to force the name to be filled, with a fallback on the
partner' parent name.

OPW-3338406

closes odoo/odoo#124825

Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>