[FIX] account_edi_ubl_cii: if no VAT PartyTaxScheme should appear

If a partner has no VAT number (for instance: a public administration),
the xml tag `PartyTaxScheme` should not appear for UBL Bis 3.

opw-3263542

closes odoo/odoo#119160

X-original-commit: 02f820fb
Signed-off-by: Laurent Smet <las@odoo.com>
Signed-off-by: Julien Van Roy <juvr@odoo.com>