Fix countryToISO() returning the local country for unknown country names

Summary:
Better to show nothing rather than something random when the country name
is not recognized or contains a typo. More importantly, this actually
enables the caller to handle this case.

Reviewers: kde-pim, winterz

Reviewed By: winterz

Subscribers: winterz

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D13002