[SPARK-8782] [SQL] Fix code generation for ORDER BY NULL

This fixes code generation for queries containing `ORDER BY NULL`.  Previously, the generated code would fail to compile.

Author: Josh Rosen <joshrosen@databricks.com>

Closes apache#7179 from JoshRosen/generate-order-fixes and squashes the following commits:

6ef49a6 [Josh Rosen] Fix ORDER BY NULL
0036696 [Josh Rosen] Add regression test for SPARK-8782 (ORDER BY NULL)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -185,6 +185,7 @@ class CodeGenContext {
     // use c1 - c2 may overflow
     case dt: DataType if isPrimitiveType(dt) => s"($c1 > $c2 ? 1 : $c1 < $c2 ? -1 : 0)"
     case BinaryType => s"org.apache.spark.sql.catalyst.util.TypeUtils.compareBinary($c1, $c2)"
+    case NullType => "0"
     case other => s"$c1.compare($c2)"
   }
 

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

@@ -1451,4 +1451,11 @@ class SQLQuerySuite extends QueryTest with BeforeAndAfterAll with SQLTestUtils {
       checkAnswer(sql("SELECT a.b FROM t ORDER BY b[0].d"), Row(Seq(Row(1))))
     }
   }
+
+  test("SPARK-8782: ORDER BY NULL") {
+    withTempTable("t") {
+      Seq((1, 2), (1, 2)).toDF("a", "b").registerTempTable("t")
+      checkAnswer(sql("SELECT * FROM t ORDER BY NULL"), Seq(Row(1, 2), Row(1, 2)))
+    }
+  }
 }