Unable to properly serialize/deserialize null values with JackJsonSerializer/Deserializer in Kotlin

**In what version(s) of Spring for Apache Kafka are you seeing this issue?**

4.0.0

**Describe the bug**

When trying to serialize a `null` value with Kotlin a runtime error occurs as the serializer can't return a `null` value, and while trying to deserialize a `null` value the code won't compile.

I will provide a pull request with failing tests and a fix