"java.lang.NullPointerException" occurs when using APPROX_QUANTILE_DS

### Affected Version
- 0.21.1
- 2 overlords, 2 coordinators
- 2 routers, 2 brokers
- 3 historicals, 11 middle managers

### Description
Hello, I am trying to calculate quantiles using "APPROX_QUANTILE_DS()".
but java.lang.NullPointerException occurs in my query.
Exception occurs in "org.apache.datasketches.quantiles.DirectUpdateDoublesSketch.growCombinedMemBuffer".

So, I think this is due to out of memory. (There is not enough memory available for the operation)
However, increasing the memory does not solve the problem.

Also, the problem only occurs when using some service codes (e.g. 'top', 'cafe')

What I'm curious about is:
1. Is it a memory problem? Or do you think there is another cause?
2. For k=128, how much memory is needed for the quantile operation?
3. Should I use a different aggregator to compute the quantile?

I don't have any good ideas to solve the problem :(

my query:
```sql
SELECT COALESCE("mytable".country, '_') AS country,
(APPROX_QUANTILE_DS("mytable".quantile_duration, 0.9)) AS quantile
FROM "mytable"
WHERE ("mytable".service_code = 'top')
AND __time >= '2021-06-01' AND __time <= '2021-06-01'
GROUP BY COALESCE("mytable".country, '_')
```

datasource configuration:
- __time : 2021-06-01/2021-06-13
- dimension: country (cardinality 239), service_code(cardinality 129) etc.
- metric: quantile_duration (Apply quantilesDoublesSketch, k=128(default)) etc.
- 7 segments (total 1.42 GB)

full log:
```shell
at org.apache.datasketches.quantiles.DirectUpdateDoublesSketch.growCombinedMemBuffer(DirectUpdateDoublesSketch.java:254)
at org.apache.datasketches.quantiles.DirectUpdateDoublesSketch.growCombinedBuffer(DirectUpdateDoublesSketch.java:238)
at org.apache.datasketches.quantiles.DoublesMergeImpl.mergeInto(DoublesMergeImpl.java:84)
at org.apache.datasketches.quantiles.DoublesUnionImpl.updateLogic(DoublesUnionImpl.java:200)
at org.apache.datasketches.quantiles.DoublesUnionImpl.update(DoublesUnionImpl.java:118)
at org.apache.druid.query.aggregation.datasketches.quantiles.DoublesSketchMergeAggregator.updateUnion(DoublesSketchMergeAggregator.java:80)
at org.apache.druid.query.aggregation.datasketches.quantiles.DoublesSketchMergeBufferAggregator.aggregate(DoublesSketchMergeBufferAggregator.java:66)
at org.apache.druid.query.aggregation.AggregatorAdapters.aggregateBuffered(AggregatorAdapters.java:164)
at org.apache.druid.query.groupby.epinephelinae.AbstractBufferHashGrouper.aggregate(AbstractBufferHashGrouper.java:161)
at org.apache.druid.query.groupby.epinephelinae.SpillingGrouper.aggregate(SpillingGrouper.java:172)
at org.apache.druid.query.groupby.epinephelinae.ConcurrentGrouper.aggregate(ConcurrentGrouper.java:269)
at org.apache.druid.query.groupby.epinephelinae.Grouper.aggregate(Grouper.java:85)
at org.apache.druid.query.groupby.epinephelinae.RowBasedGrouperHelper.lambda$createGrouperAccumulatorPair$2(RowBasedGrouperHelper.java:332)
at org.apache.druid.java.util.common.guava.MappingAccumulator.accumulate(MappingAccumulator.java:40)
at org.apache.druid.java.util.common.guava.BaseSequence.accumulate(BaseSequence.java:44)
at org.apache.druid.java.util.common.guava.ConcatSequence.lambda$accumulate$0(ConcatSequence.java:41)
at org.apache.druid.java.util.common.guava.MappingAccumulator.accumulate(MappingAccumulator.java:40)
at org.apache.druid.java.util.common.guava.FilteringAccumulator.accumulate(FilteringAccumulator.java:41)
at org.apache.druid.java.util.common.guava.MappingAccumulator.accumulate(MappingAccumulator.java:40)
at org.apache.druid.java.util.common.guava.BaseSequence.accumulate(BaseSequence.java:44)
at org.apache.druid.java.util.common.guava.MappedSequence.accumulate(MappedSequence.java:43)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.java.util.common.guava.SequenceWrapper.wrap(SequenceWrapper.java:55)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.java.util.common.guava.FilteredSequence.accumulate(FilteredSequence.java:45)
at org.apache.druid.java.util.common.guava.MappedSequence.accumulate(MappedSequence.java:43)
at org.apache.druid.java.util.common.guava.ConcatSequence.accumulate(ConcatSequence.java:41)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.java.util.common.guava.SequenceWrapper.wrap(SequenceWrapper.java:55)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.java.util.common.guava.SequenceWrapper.wrap(SequenceWrapper.java:55)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.java.util.common.guava.LazySequence.accumulate(LazySequence.java:40)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.java.util.common.guava.SequenceWrapper.wrap(SequenceWrapper.java:55)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.java.util.common.guava.MappedSequence.accumulate(MappedSequence.java:43)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.java.util.common.guava.SequenceWrapper.wrap(SequenceWrapper.java:55)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.java.util.common.guava.LazySequence.accumulate(LazySequence.java:40)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.java.util.common.guava.SequenceWrapper.wrap(SequenceWrapper.java:55)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.query.spec.SpecificSegmentQueryRunner$1.accumulate(SpecificSegmentQueryRunner.java:87)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.query.spec.SpecificSegmentQueryRunner.doNamed(SpecificSegmentQueryRunner.java:171)
at org.apache.druid.query.spec.SpecificSegmentQueryRunner.access$100(SpecificSegmentQueryRunner.java:44)
at org.apache.druid.query.spec.SpecificSegmentQueryRunner$2.wrap(SpecificSegmentQueryRunner.java:153)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.java.util.common.guava.WrappingSequence$1.get(WrappingSequence.java:50)
at org.apache.druid.query.CPUTimeMetricQueryRunner$1.wrap(CPUTimeMetricQueryRunner.java:78)
at org.apache.druid.java.util.common.guava.WrappingSequence.accumulate(WrappingSequence.java:45)
at org.apache.druid.query.groupby.epinephelinae.GroupByMergingQueryRunnerV2$1$1$1.call(GroupByMergingQueryRunnerV2.java:247)
at org.apache.druid.query.groupby.epinephelinae.GroupByMergingQueryRunnerV2$1$1$1.call(GroupByMergingQueryRunnerV2.java:234)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at org.apache.druid.query.PrioritizedListenableFutureTask.run(PrioritizedExecutorService.java:247)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
```

Any help would be greatly appreciated.

Start by reproducing the APPROX_QUANTILE_DS query described in the issue and trace the stack through DoublesSketchMergeAggregator.java and DoublesSketchMergeBufferAggregator.java to DirectUpdateDoublesSketch.growCombinedMemBuffer. Compare behavior for the reported service codes and k=128. Done means the cause of the NullPointerException is identified and a regression test covers the failing case.

Written by the indexing model from the issue text.