vat_state should not be required even if vat_number is set in create_pay_link

[rejoc]

In function create_pay_link, if vat_number is set and vat_state is not, we get an error ValueError('vat_state must be set if vat_number is set')

According to https://developer.paddle.com/api-reference/product-api/pay-links/createpaylink this field is NOT required (some countries don't have states...).