collect_set should ignore null values

select collect_set(null) from (select 1) tmp;

java.lang.IllegalArgumentException: Flat hash tables cannot contain null elements.
	at scala.collection.mutable.FlatHashTable$HashUtils$class.elemHashCode(FlatHashTable.scala:390)
	at scala.collection.mutable.HashSet.elemHashCode(HashSet.scala:41)
	at scala.collection.mutable.FlatHashTable$class.addEntry(FlatHashTable.scala:136)
	at scala.collection.mutable.HashSet.addEntry(HashSet.scala:41)
	at scala.collection.mutable.HashSet.$plus$eq(HashSet.scala:60)
	at scala.collection.mutable.HashSet.$plus$eq(HashSet.scala:41)
	at org.apache.spark.sql.catalyst.expressions.aggregate.Collect.update(collect.scala:64)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$1$$anonfun$applyOrElse$1.apply(AggregationIterator.scala:170)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$1$$anonfun$applyOrElse$1.apply(AggregationIterator.scala:170)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$generateProcessRow$1.apply(AggregationIterator.scala:186)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$generateProcessRow$1.apply(AggregationIterator.scala:180)
	at org.apache.spark.sql.execution.aggregate.SortBasedAggregationIterator.processCurrentSortedGroup(SortBasedAggregationIterator.scala:115)
	at org.apache.spark.sql.execution.aggregate.SortBasedAggregationIterator.next(SortBasedAggregationIterator.scala:150)
	at org.apache.spark.sql.execution.aggregate.SortBasedAggregationIterator.next(SortBasedAggregationIterator.scala:29)
	at org.apache.spark.sql.execution.SparkPlan$$anonfun$3.apply(SparkPlan.scala:232)
	at org.apache.spark.sql.execution.SparkPlan$$anonfun$3.apply(SparkPlan.scala:225)