Null value throws error later on

[baptisteghesquiere]

https://github.com/OCA/l10n-belgium/blob/cce11150149038f83b393bc1787c94d2208e808f/companyweb_base/models/res_partner.py#L291

When the value of CountryCode is empty, this function fails because in the function you do a upper(), but you can not perform an upper() on a null value.

This should be better handled as this throws an awfull stack trace error.