Handle {} as a default value in IDL

[julianpeeters]

The {} in ME last_meta_event = {}; results in new ME(, , ), which is incorrect. Check the behavior of Apache Avro Java compilation and conform to that as the defacto standard, wether it throws an error or treats it as a 0-arg ctor to generate new ME("asd", "sdf", None).

Example Idl:

@namespace("com.google.analytics")
protocol MEProtocol {
record ME {
    string type = "asd";
    string service = "sdf";
    union { null, string } service_timezone = null;
}
record Visit {
    ME first_meta_event = {"type": "asd", "service": "sdf", "service_timezone": null };
    ME last_meta_event = {};
  }
}