Adding the null check in combine and fold in doublesSketch

[pranavbhole]

Fixes #14520.

Description

Adding the null check in combine and fold in doublesSketch, added the test for fold and combine.

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) ~[?:?]

Fixed the bug ...

Renamed the class ...

Added a forbidden-apis entry ...

Release note

---

Key changed/added classes in this PR

• MyFoo
• OurBar
• TheirBaz

---

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.

[pranavbhole]

@abhishekagarwal87 @clintropolis Please review
Thank you