When IP geolocation returned no matching CIDR block, LookupCountryByIP
returned an empty string with nil error. The handler took the address of
that empty string, producing a non-nil pointer to "", which was inserted
into the database. Guard against this by returning nil when the resolved
country code is empty, and backfill existing rows with a migration.
Signed-off-by: Émile Ré <emile@getprobo.com>
ecb9bd747e