Fixed null optional strings serialized to "null"

node-protobuf currently serializes null strings as "null". I think this is error prone and a better solution would be to ignore such a field.

Unfortunately (unless I am mistaken), protobuf does not have a semantic for null values, making this solution only valid for optional fields.

This fix does not handle required fields. I suppose the correct behaviour would be to throw an exception in case a required field is set to null.