public static class Serializers.DummyAvroRegisteredClass
This is used in case we don't have Avro on the classpath. Flink versions before 1.4 always
registered special Serializers for Kryo but starting with Flink 1.4 we don't have Avro on the
classpath by default anymore. We still have to retain the same registered Serializers for
backwards compatibility of savepoints.