Fix logic for countries that don't have mobile-begin-with

[A D Snook at Automattic]

If you call the module with a phone number but not a country, and the phone number includes a country code (e.g. +54 for Argentina) and the country doesn't have any mobile_begin_with values, then the internal function get_iso3166_by_phone will fail to return an iso3166 entry and valid phone numbers for that country will fail to validate.