Fix country_id requirement in partner

⚠ TODO -> revert this commit when all partners in production do have a filled country_id field. ⚠

Country_id is required for VAT validation but cannot be required because so far we have partners in odoo production that were imported from tryton without a country_id field.

With country_id required, these partners are blocked in production from any action around their view (wizards, buttons to access contracts, activities...)