[SPARK-17641][SQL] Collect_list/Collect_set should not collect null values.

## What changes were proposed in this pull request?
We added native versions of `collect_set` and `collect_list` in Spark 2.0. These currently also (try to) collect null values, this is different from the original Hive implementation. This PR fixes this by adding a null check to the `Collect.update` method.

## How was this patch tested?
Added a regression test to `DataFrameAggregateSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #15208 from hvanhovell/SPARK-17641.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/collect.scala

@@ -65,7 +65,12 @@ abstract class Collect extends ImperativeAggregate {
   }
 
   override def update(b: MutableRow, input: InternalRow): Unit = {
-    buffer += child.eval(input)
+    // Do not allow null values. We follow the semantics of Hive's collect_list/collect_set here.
+    // See: org.apache.hadoop.hive.ql.udf.generic.GenericUDAFMkCollectionEvaluator
+    val value = child.eval(input)
+    if (value != null) {
+      buffer += value
+    }
   }
 
   override def merge(buffer: MutableRow, input: InternalRow): Unit = {

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

@@ -477,6 +477,18 @@ class DataFrameAggregateSuite extends QueryTest with SharedSQLContext {
     assert(error.message.contains("collect_set() cannot have map type data"))
   }
 
+  test("SPARK-17641: collect functions should not collect null values") {
+    val df = Seq(("1", 2), (null, 2), ("1", 4)).toDF("a", "b")
+    checkAnswer(
+      df.select(collect_list($"a"), collect_list($"b")),
+      Seq(Row(Seq("1", "1"), Seq(2, 2, 4)))
+    )
+    checkAnswer(
+      df.select(collect_set($"a"), collect_set($"b")),
+      Seq(Row(Seq("1"), Seq(2, 4)))
+    )
+  }
+
   test("SPARK-14664: Decimal sum/avg over window should work.") {
     checkAnswer(
       spark.sql("select sum(a) over () from values 1.0, 2.0, 3.0 T(a)"),