[IMP] l10n_it_edi: tax representative should have vat number

If Company Tax Representative of the seller is not having a VAT number,
which is mandatory, system won't allow to validate the invoice.

closes odoo/odoo#34511

Task: 1971512
Closes: #34511
Signed-off-by: Josse Colpaert <jco@openerp.com>