when we declare some fields in a record, it's assumed to be mandatory unless
we specify it with a union {original_type, null}.this feature is actually very useful to me: I declare a schema, and if some new user creates a record, but fails to populate some mandatory field, I want some mechanism to throw an exception. currently I get this feature "by accident": if the user misses some mandatory fields, when I send this record as a param to some RPC method, or when I serialize it through SpecificDatumWriter, Avro throws a NULLPointerException. I can catch this and let user know that it's due to lack of mandatory fields. but instead of NPE, could we make Avro throw a more explicit exception, so it's more descriptive? Thanks Yang
