[SPARK-15204][SQL] improve nullability inference for Aggregator

## What changes were proposed in this pull request?

TypedAggregateExpression sets nullable based on the schema of the outputEncoder

## How was this patch tested?

Add test in DatasetAggregatorSuite

Author: Koert Kuipers <koert@tresata.com>

Closes #13532 from koertkuipers/feat-aggregator-nullable.

sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/TypedAggregateExpression.scala

@@ -51,7 +51,8 @@ object TypedAggregateExpression {
       bufferDeserializer,
       outputEncoder.serializer,
       outputEncoder.deserializer.dataType,
-      outputType)
+      outputType,
+      !outputEncoder.flat || outputEncoder.schema.head.nullable)
   }
 }
 
@@ -65,9 +66,8 @@ case class TypedAggregateExpression(
     bufferDeserializer: Expression,
     outputSerializer: Seq[Expression],
     outputExternalType: DataType,
-    dataType: DataType) extends DeclarativeAggregate with NonSQLExpression {
-
-  override def nullable: Boolean = true
+    dataType: DataType,
+    nullable: Boolean) extends DeclarativeAggregate with NonSQLExpression {
 
   override def deterministic: Boolean = true
 

sql/core/src/test/scala/org/apache/spark/sql/DatasetAggregatorSuite.scala

@@ -305,4 +305,13 @@ class DatasetAggregatorSuite extends QueryTest with SharedSQLContext {
     val ds = Seq(1, 2, 3).toDS()
     checkDataset(ds.select(MapTypeBufferAgg.toColumn), 1)
   }
+
+  test("SPARK-15204 improve nullability inference for Aggregator") {
+    val ds1 = Seq(1, 3, 2, 5).toDS()
+    assert(ds1.select(typed.sum((i: Int) => i)).schema.head.nullable === false)
+    val ds2 = Seq(AggData(1, "a"), AggData(2, "a")).toDS()
+    assert(ds2.select(SeqAgg.toColumn).schema.head.nullable === true)
+    val ds3 = sql("SELECT 'Some String' AS b, 1279869254 AS a").as[AggData]
+    assert(ds3.select(NameAgg.toColumn).schema.head.nullable === true)
+  }
 }