DataSketches aggregator NPEs post upgrade to 4.0.0

[gianm]

After upgrading to datasketches 4.0.0 (#14334) we are seeing an NPE during ingestion. Stack trace below.

The issue is union.update(sketch) in DataSketches 3.x accepts null sketch, but union.union(sketch) in DataSketches 4.x doesn't. Here's the code from DataSketches 3.3.0: https://github.com/apache/datasketches-java/blob/3.3.0/src/main/java/org/apache/datasketches/quantiles/DoublesUnionImpl.java#L116-L119

I think we can fix this by skipping the call to union.union(sketch) if sketch is null. We also need to:

• audit other calls to DoublesUnion#union for possible null inputs and fix those too.
• audit other sketches for similar situations (where nulls used to be accepted and are no longer) and fix those up if there are any.
• add tests for these cases.

2023-07-03T13:59:20,448 ERROR [task-runner-0-priority-0] org.apache.druid.indexing.overlord.SingleTaskBackgroundRunner - Exception while running task[<task>]
java.lang.NullPointerException: null
	at java.util.Objects.requireNonNull(Objects.java:209) ~[?:?]
	at org.apache.datasketches.quantiles.DoublesUnionImpl.union(DoublesUnionImpl.java:124) ~[datasketches-java-4.0.0.jar:?]
	at org.apache.druid.query.aggregation.datasketches.quantiles.DoublesSketchAggregatorFactory$2.fold(DoublesSketchAggregatorFactory.java:266) ~[?:?]
	at org.apache.druid.query.aggregation.datasketches.quantiles.DoublesSketchAggregatorFactory$2.reset(DoublesSketchAggregatorFactory.java:259) ~[?:?]
	at org.apache.druid.segment.RowCombiningTimeAndDimsIterator.resetCombinedMetrics(RowCombiningTimeAndDimsIterator.java:249) ~[druid-processing.jar]
	at org.apache.druid.segment.RowCombiningTimeAndDimsIterator.combineToCurrentTimeAndDims(RowCombiningTimeAndDimsIterator.java:229) ~[druid-processing.jar]
	at org.apache.druid.segment.RowCombiningTimeAndDimsIterator.moveToNext(RowCombiningTimeAndDimsIterator.java:191) ~[druid-processing.jar]
	at org.apache.druid.segment.IndexMergerV9.mergeIndexesAndWriteColumns(IndexMergerV9.java:605) ~[druid-processing.jar]
	at org.apache.druid.segment.IndexMergerV9.makeIndexFiles(IndexMergerV9.java:233) ~[druid-processing.jar]
	at org.apache.druid.segment.IndexMergerV9.merge(IndexMergerV9.java:1155) ~[druid-processing.jar]
	at org.apache.druid.segment.IndexMergerV9.multiphaseMerge(IndexMergerV9.java:972) ~[druid-processing.jar]
	at org.apache.druid.segment.IndexMergerV9.mergeQueryableIndex(IndexMergerV9.java:914) ~[druid-processing.jar]
	at org.apache.druid.indexing.common.task.batch.parallel.PartialSegmentMergeTask.mergeSegmentsInSamePartition(PartialSegmentMergeTask.java:352) ~[druid-indexing-service.jar]
	at org.apache.druid.indexing.common.task.batch.parallel.PartialSegmentMergeTask.mergeAndPushSegments(PartialSegmentMergeTask.java:260) ~[druid-indexing-service.jar]
	at org.apache.druid.indexing.common.task.batch.parallel.PartialSegmentMergeTask.runTask(PartialSegmentMergeTask.java:191) ~[druid-indexing-service.jar]
	at org.apache.druid.indexing.common.task.batch.parallel.PartialGenericSegmentMergeTask.runTask(PartialGenericSegmentMergeTask.java:46) ~[druid-indexing-service.jar]
	at org.apache.druid.indexing.common.task.AbstractTask.run(AbstractTask.java:173) ~[druid-indexing-service.jar]
	at org.apache.druid.indexing.overlord.SingleTaskBackgroundRunner$SingleTaskBackgroundRunnerCallable.call(SingleTaskBackgroundRunner.java:477) ~[druid-indexing-service.jar]
	at org.apache.druid.indexing.overlord.SingleTaskBackgroundRunner$SingleTaskBackgroundRunnerCallable.call(SingleTaskBackgroundRunner.java:449) ~[druid-indexing-service.jar]
	at java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[?:?]
	at java.lang.Thread.run(Thread.java:833) ~[?:?]