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.