blaise nullable message value

[mishabruml]

Hi there,

As i understand it, the message value is nullable https://github.com/ovotech/castle/blob/193347cd0c3f30a1565a464da6b0942de089af13/packages/avro-kafkajs/src/types.ts#L37 but when overriding message value using blaise, typescript compiler complains

meaning that I've had to do { value: null as unknown as undefined} for now 🤢