Fixes: iOS 7 returns @"" instead of nil if sim card is missing

iOS 7 returns on isoCountryCode a empty string, the documentation says
it should return nil on missing sim card.
iOS 8 behaves correctly and returns nil if no sim card is present.
Merge pull request #31 from hashier/fix/isoCountryOniOS7

Fixes: iOS 7 returns @"" instead of nil if sim card is missing