The API supportDataType in FileFormat helps to validate the output/input schema before exection starts. So that we can avoid some invalid data source IO, and users can see clean error messages.
This PR is to override the validation API in Avro data source.
Also, as per the spec of Avro(https://avro.apache.org/docs/1.8.2/spec.html), NullType is supported. This PR fixes the handling of NullType.