Don't include the locale country if no country name is provided

Straight from the docs:

> When `NO` and address includes a country name, that country name is still included in the return value.
> When `YES` and address doesn’t include a country name, the country name is added to the return value. (The country name is generated from the country code entry in address; see "Address Property" in ABPerson Reference.)

The user of this formatter almost certainly doesn't want an empty address generating a non-empty string. Let's give them an empty string!