[FIX] account_edi_ubl_cii: fix invoice import

Sometimes the invoice import fails, because `payment_terms_node.text`
can be None and we're trying to concatenate it with a string. This
commit makes sure that `payment_terms_node.text` is there first.

task-3132980

closes odoo/odoo#114329

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