[FIX] website_portal_sale: VAT check
[FIX] website_portal_sale: VAT check

- Create a SO for a customer (without VAT), validate the invoice
- Connected as the customer, go to 'my/account', and change any
  information (e.g. phone number).

You receive the error 'Changing VAT number is not allowed...'.

This is because `data.get('vat', partner.vat)` is `u''`, while
`partner.vat` is `False`.

opw-696962

ygol/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,dfang/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo