[FIX] point_of_sale: ensure customer presence for `split_transactions`

This commit addresses an issue where `split_transactions` payments were
allowed to be added without adding a customer during the creation of
return orders. The problem led to errors during session closure.

The solution is that in the backend when creating return orders and
adding payments, enforce the presence of a customer for
`split_transactions` by showing an error.

opw-3468160

closes odoo/odoo#132004

Signed-off-by: David Monnom (moda) <moda@odoo.com>