[SPARK-19218][SC-6086] SET -v should not cause a NPE when a configuration uses NULL as its default value

This is a backport of apache@c4a6519

#### What changes were proposed in this pull request?
This PR aims to fix the following two things:

1. `sql("SET -v").collect()` or `sql("SET -v").show()` throws a NullPointerException when a String configuration with default value `null` has been defined.
2. Currently, `SET` and `SET -v` commands show unsorted result. This PR sorts the result.

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

Author: Herman van Hovell <[email protected]>
Author: Dongjoon Hyun <[email protected]>

Closes apache#265 from hvanhovell/SPARK-19218.

Author: hvanhovell

sql/core/src/main/scala/org/apache/spark/sql/execution/command/SetCommand.scala

@@ -79,16 +79,17 @@ case class SetCommand(kv: Option[(String, Option[String])]) extends RunnableComm
     // Queries all key-value pairs that are set in the SQLConf of the sparkSession.
     case None =>
       val runFunc = (sparkSession: SparkSession) => {
-        sparkSession.conf.getAll.map { case (k, v) => Row(k, v) }.toSeq
+        sparkSession.conf.getAll.toSeq.sorted.map { case (k, v) => Row(k, v) }
       }
       (keyValueOutput, runFunc)
 
     // Queries all properties along with their default values and docs that are defined in the
     // SQLConf of the sparkSession.
     case Some(("-v", None)) =>
       val runFunc = (sparkSession: SparkSession) => {
-        sparkSession.sessionState.conf.getAllDefinedConfs.map { case (key, defaultValue, doc) =>
-          Row(key, defaultValue, doc)
+        sparkSession.sessionState.conf.getAllDefinedConfs.sorted.map {
+          case (key, defaultValue, doc) =>
+            Row(key, Option(defaultValue).getOrElse("<undefined>"), doc)
         }
       }
       val schema = StructType(

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

@@ -985,6 +985,33 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
     spark.sessionState.conf.clear()
   }
 
+  test("SPARK-19218 SET command should show a result in a sorted order") {
+    val overrideConfs = sql("SET").collect()
+    sql(s"SET test.key3=1")
+    sql(s"SET test.key2=2")
+    sql(s"SET test.key1=3")
+    val result = sql("SET").collect()
+    assert(result ===
+      (overrideConfs ++ Seq(
+        Row("test.key1", "3"),
+        Row("test.key2", "2"),
+        Row("test.key3", "1"))).sortBy(_.getString(0))
+    )
+    spark.sessionState.conf.clear()
+  }
+
+  test("SPARK-19218 `SET -v` should not fail with null value configuration") {
+    import SQLConf._
+    val confEntry = buildConf("spark.test").doc("doc").stringConf.createWithDefault(null)
+
+    try {
+      val result = sql("SET -v").collect()
+      assert(result === result.sortBy(_.getString(0)))
+    } finally {
+      SQLConf.unregister(confEntry)
+    }
+  }
+
   test("SET commands with illegal or inappropriate argument") {
     spark.sessionState.conf.clear()
     // Set negative mapred.reduce.tasks for automatically determining