Re-enable NullType sorting test now that SPARK-8868 is fixed

Author: JoshRosen

sql/core/src/test/scala/org/apache/spark/sql/execution/UnsafeExternalSortSuite.scala

@@ -38,7 +38,7 @@ class UnsafeExternalSortSuite extends SparkPlanTest with BeforeAndAfterAll {
 
   // Test sorting on different data types
   for (
-    dataType <- DataTypeTestUtils.atomicTypes // Disable null type for now due to bug in SqlSerializer2 ++ Set(NullType);
+    dataType <- DataTypeTestUtils.atomicTypes ++ Set(NullType)
     if !dataType.isInstanceOf[DecimalType]; // Since we don't have an unsafe representation for decimals
     nullable <- Seq(true, false);
     sortOrder <- Seq('a.asc :: Nil, 'a.desc :: Nil);