[FIX] base_vat: keep leading zeros on San Marino tax id

### Steps to reproduce
* install the *VAT Number Validation `(base_vat)`* and Contacts module.
* Create a new contact from San Marino and set their Tax Id to SM05426
* Save the contact

You should see that the leading zeros have been removed on the Tax Id
(here, SM05426 became SM5426)

opw-3007722

closes odoo/odoo#105828

X-original-commit: 24207914
Signed-off-by: Josse Colpaert <jco@odoo.com>