KafkaConfig value sasl.server.callback.handler.class is set to null even when custom class is used

As mentioned in the summary, the value `sasl.server.callback.handler.class` is set to `null` in the terminal printout of the KafkaConfig object, even though a custom handler class has been configured, and has been confirmed to be used by Kafka (by testing the callback's behavior).

This issue can (has) lead to hours of unnecessary debugging, as well as potential security issues, since not knowing if your brokers are using, for example, the insecure SASL/OAUTHBEARER default handler bundled with Kafka, or some custom implementation, can lead to security breaches.