MINOR: Add null check to array type fields in toString generation code

[omkreddy]

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[omkreddy]

Observed below NullPointerExceptions in debug logs:

SLF4J: Failed toString() invocation on an object of type [org.apache.kafka.common.requests.MetadataRequest$Builder]
Reported exception:
java.lang.NullPointerException
	at org.apache.kafka.common.message.MetadataRequestData.toString(MetadataRequestData.java:292)
	at org.apache.kafka.common.requests.MetadataRequest$Builder.toString(MetadataRequest.java:100)
	at org.slf4j.helpers.MessageFormatter.safeObjectAppend(MessageFormatter.java:299)
	at org.slf4j.helpers.MessageFormatter.deeplyAppendParameter(MessageFormatter.java:271)
	at org.slf4j.helpers.MessageFormatter.arrayFormat(MessageFormatter.java:233)
	at org.slf4j.helpers.MessageFormatter.arrayFormat(MessageFormatter.java:173)
	at org.apache.kafka.common.utils.LogContext$LocationAwareKafkaLogger.writeLog(LogContext.java:428)
	at org.apache.kafka.common.utils.LogContext$LocationAwareKafkaLogger.debug(LogContext.java:222)
	at org.apache.kafka.clients.NetworkClient$DefaultMetadataUpdater.maybeUpdate(NetworkClient.java:1073)
	at org.apache.kafka.clients.NetworkClient$DefaultMetadataUpdater.maybeUpdate(NetworkClient.java:982)
	at org.apache.kafka.clients.NetworkClient.poll(NetworkClient.java:533)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:262)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:233)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:224)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.awaitMetadataUpdate(ConsumerNetworkClient.java:161)
	at org.apache.kafka.clients.consumer.internals.AbstractCoordinator.ensureCoordinatorReady(AbstractCoordinator.java:245)
	at io.confluent.security.store.kafka.coordinator.MetadataServiceCoordinator.poll(MetadataServiceCoordinator.java:145)
	at io.confluent.security.store.kafka.coordinator.MetadataNodeManager.run(MetadataNodeManager.java:132)

[cmccabe]

Thanks @omkreddy . Check out #6489 -- it includes a fix for this and another issue related to deserialization.

[omkreddy]

Closing in favor of #6489 .