Attempt to allow null and empty country objects

Possible fix for #82. I'm not totally certain I got everything right, as we might want to add more tests to verify behavior is working appropriately. The biggest thing to check here might be the round-tripping of both `None` and `u''` values, to make sure the code attribute is set appropriately on the returned Country object from the descriptor.
What's the point in this change? It doesn't seem very 2-3 compatible.