[FIX] l10n_es_edi_sii: partners without vat should work as well

Before, the system required a vat number on every partner, but that
is not required.  We need to send it as some other kind of ID however.

To clarify, a partner without vat is not for the simplified case only.

closes odoo/odoo#81906

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