avro

[AVRO-2278] getter semantics confusing. (#864)

* [fix] getter semantics confusing.

impossible to distinguish between a correct field with the value null.
and a nonsense field.

* [fix]delete soem generated class

* [fix]delete soem generated class

* [fix]delete soem generated class

* [add] sync up exception behavior for consistency, and better messages.

* [add] helper method.