[SPARK-4319][SQL] Enable an ignored test "null count".

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3185 from ueshin/issues/SPARK-4319 and squashes the following commits:

a44a38e [Takuya UESHIN] Enable an ignored test "null count".

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

@@ -281,14 +281,13 @@ class SQLQuerySuite extends QueryTest with BeforeAndAfterAll {
       3)
   }
 
-  // No support for primitive nulls yet.
-  ignore("null count") {
+  test("null count") {
     checkAnswer(
-      sql("SELECT a, COUNT(b) FROM testData3"),
-      Seq((1,0), (2, 1)))
+      sql("SELECT a, COUNT(b) FROM testData3 GROUP BY a"),
+      Seq((1, 0), (2, 1)))
 
     checkAnswer(
-      testData3.groupBy()(Count('a), Count('b), Count(1), CountDistinct('a :: Nil), CountDistinct('b :: Nil)),
+      sql("SELECT COUNT(a), COUNT(b), COUNT(1), COUNT(DISTINCT a), COUNT(DISTINCT b) FROM testData3"),
       (2, 1, 2, 2, 1) :: Nil)
   }
 

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

@@ -64,11 +64,12 @@ object TestData {
       BinaryData("123".getBytes(), 4) :: Nil).toSchemaRDD
   binaryData.registerTempTable("binaryData")
 
-  // TODO: There is no way to express null primitives as case classes currently...
+  case class TestData3(a: Int, b: Option[Int])
   val testData3 =
-    logical.LocalRelation('a.int, 'b.int).loadData(
-      (1, null) ::
-      (2, 2) :: Nil)
+    TestSQLContext.sparkContext.parallelize(
+      TestData3(1, None) ::
+      TestData3(2, Some(2)) :: Nil).toSchemaRDD
+  testData3.registerTempTable("testData3")
 
   val emptyTableData = logical.LocalRelation('a.int, 'b.int)