HiveUDAF should return NULL in case of 0 rows

When query returns zero rows, the HiveUDAFFunction throws NPE

CASE 1:
create table abc(a int)
select histogram_numeric(a,2) from abc // NPE

Job aborted due to stage failure: Task 0 in stage 1.0 failed 1 times, most recent failure: Lost task 0.0 in stage 1.0 (TID 0, localhost, executor driver): java.lang.NullPointerException
at org.apache.spark.sql.hive.HiveUDAFFunction.eval(hiveUDFs.scala:471)
at org.apache.spark.sql.hive.HiveUDAFFunction.eval(hiveUDFs.scala:315)
at org.apache.spark.sql.catalyst.expressions.aggregate.TypedImperativeAggregate.eval(interfaces.scala:543)
at org.apache.spark.sql.execution.aggregate.AggregationIterator.$anonfun$generateResultProjection$5(AggregationIterator.scala:231)
at org.apache.spark.sql.execution.aggregate.ObjectAggregationIterator.outputForEmptyGroupingKeyWithoutInput(ObjectAggregationIterator.scala:97)
at org.apache.spark.sql.execution.aggregate.ObjectHashAggregateExec.$anonfun$doExecute$2(ObjectHashAggregateExec.scala:132)
at org.apache.spark.sql.execution.aggregate.ObjectHashAggregateExec.$anonfun$doExecute$2$adapted(ObjectHashAggregateExec.scala:107)
at org.apache.spark.rdd.RDD.$anonfun$mapPartitionsWithIndexInternal$2(RDD.scala:839)
at org.apache.spark.rdd.RDD.$anonfun$mapPartitionsWithIndexInternal$2$adapted(RDD.scala:839)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:327)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:291)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:327)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:291)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:327)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:291)
at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:90)
at org.apache.spark.scheduler.Task.run(Task.scala:122)
at org.apache.spark.executor.Executor$TaskRunner.$anonfun$run$3(Executor.scala:425)
at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1350)
at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:428)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

CASE 2:

create table abc(a int)
insert into abc values (1)
select histogram_numeric(a,2) from abc where a=3 //NPE

Job aborted due to stage failure: Task 0 in stage 4.0 failed 1 times, most recent failure: Lost task 0.0 in stage 4.0 (TID 5, localhost, executor driver): java.lang.NullPointerException
at org.apache.spark.sql.hive.HiveUDAFFunction.serialize(hiveUDFs.scala:477)
at org.apache.spark.sql.hive.HiveUDAFFunction.serialize(hiveUDFs.scala:315)
at org.apache.spark.sql.catalyst.expressions.aggregate.TypedImperativeAggregate.serializeAggregateBufferInPlace(interfaces.scala:570)
at org.apache.spark.sql.execution.aggregate.AggregationIterator.$anonfun$generateResultProjection$6(AggregationIterator.scala:254)
at org.apache.spark.sql.execution.aggregate.ObjectAggregationIterator.outputForEmptyGroupingKeyWithoutInput(ObjectAggregationIterator.scala:97)
at org.apache.spark.sql.execution.aggregate.ObjectHashAggregateExec.$anonfun$doExecute$2(ObjectHashAggregateExec.scala:132)
at org.apache.spark.sql.execution.aggregate.ObjectHashAggregateExec.$anonfun$doExecute$2$adapted(ObjectHashAggregateExec.scala:107)
at org.apache.spark.rdd.RDD.$anonfun$mapPartitionsWithIndexInternal$2(RDD.scala:839)
at org.apache.spark.rdd.RDD.$anonfun$mapPartitionsWithIndexInternal$2$adapted(RDD.scala:839)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:327)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:291)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:327)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:291)
at org.apache.spark.shuffle.ShuffleWriteProcessor.write(ShuffleWriteProcessor.scala:59)
at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:94)
at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:52)
at org.apache.spark.scheduler.Task.run(Task.scala:122)
at org.apache.spark.executor.Executor$TaskRunner.$anonfun$run$3(Executor.scala:425)
at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1350)
at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:428)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)