[FIX] account_edi_ubl_cii: import customer when uploading an invoice

Previously, it was only possible to upload a bill, so the partner_id was read
from the vendor in the imported file. Now, it is also possible to upload a
file to create an invoice, so we need to be able to decode the customer in
the imported file.

opw-3072989

closes odoo/odoo#106765

Signed-off-by: Laurent Smet <las@odoo.com>