[FIX] base_vat: Fix vat number starting with 'EU' for not-EU companies

Foreign companies that trade with non-enterprises in the EU may have a VATIN starting with "EU" instead of a country code.

Currently, the user is facing a traceback in this situation since stdnum doesn't have any format method for vat number starting with 'EU'.

closes odoo/odoo#93463

Ticket: 2877716
Signed-off-by: Olivier Colson (oco) <oco@odoo.com>