Add check for required avroBytesDecoder property that otherwise causes NPE.

[abhishekrb19]

This PR fixes an NPE that occurs when avroBytesDecoder is missing in the sampling spec. This is the default behavior from the Druid web console when sampling Avro Kafka streams, unless you manually edit the spec to add in the required property.

The NPE stacktrace:

Caused by: java.lang.NullPointerException: Cannot invoke "org.apache.druid.data.input.avro.AvroBytesDecoder.parse(java.nio.ByteBuffer)" because "this.avroBytesDecoder" is null
	at org.apache.druid.data.input.avro.AvroStreamReader.intermediateRowIterator(AvroStreamReader.java:78) ~[?:?]
	at org.apache.druid.data.input.IntermediateRowParsingReader.intermediateRowIteratorWithMetadata(IntermediateRowParsingReader.java:231) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.data.input.IntermediateRowParsingReader.sample(IntermediateRowParsingReader.java:124) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.data.input.impl.InputEntityIteratingReader.lambda$sample$1(InputEntityIteratingReader.java:98) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.java.util.common.parsers.CloseableIterator$2.findNextIteratorIfNecessary(CloseableIterator.java:84) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.java.util.common.parsers.CloseableIterator$2.<init>(CloseableIterator.java:69) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.java.util.common.parsers.CloseableIterator.flatMap(CloseableIterator.java:67) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.data.input.impl.InputEntityIteratingReader.createIterator(InputEntityIteratingReader.java:108) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.data.input.impl.InputEntityIteratingReader.sample(InputEntityIteratingReader.java:94) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.data.input.impl.TimedShutoffInputSourceReader.sample(TimedShutoffInputSourceReader.java:62) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.segment.transform.TransformingInputSourceReader.sample(TransformingInputSourceReader.java:50) ~[druid-processing-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]
	at org.apache.druid.indexing.overlord.sampler.InputSourceSampler.sample(InputSourceSampler.java:129) ~[druid-indexing-service-2023.05.0-iap-SNAPSHOT.jar:2023.05.0-iap-SNAPSHOT]

Screenshot before the fix:

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[zachjsh]

🚀

[abhishekrb19]

I see some unrelated test failures:

Error:  Errors: 
Error:  org.apache.druid.client.HttpServerInventoryViewTest.testSyncMonitoring
Error:    Run 1: HttpServerInventoryViewTest.testSyncMonitoring:330 » RejectedExecution Service not started.
Error:    Run 2: HttpServerInventoryViewTest.testSyncMonitoring:330 » RejectedExecution Service not started.
Error:    Run 3: HttpServerInventoryViewTest.testSyncMonitoring:330 » RejectedExecution Service not started.
Error:    Run 4: HttpServerInventoryViewTest.testSyncMonitoring:330 » RejectedExecution Service not started.

They appear to be fixed in #14180