If you look at the updateState API of storm.kafka.trident.TridentKafkaState. When producer is sending data its not handling if the null value is sent by mapper.getMessageFromTuple(tuple). Results into Kafka topic gets value as "null" string. There might be case in particular kind of exception user do not want to replay tuple and just report it and with that he needs to return null.
Also make the members as protected as I need to copy-paste the class to provide my implementation.
My updateState API looks like this