[SPARK-20872][SQL] ShuffleExchange.nodeName should handle null coordinator

## What changes were proposed in this pull request?

A one-liner change in `ShuffleExchange.nodeName` to cover the case when `coordinator` is `null`, so that the match expression is exhaustive.

Please refer to [SPARK-20872](https://issues.apache.org/jira/browse/SPARK-20872) for a description of the symptoms.
TL;DR is that inspecting a `ShuffleExchange` (directly or transitively) on the Executor side can hit a case where the `coordinator` field of a `ShuffleExchange` is null, and thus will trigger a `MatchError` in `ShuffleExchange.nodeName()`'s inexhaustive match expression.

Also changed two other match conditions in `ShuffleExchange` on the `coordinator` field to be consistent.

## How was this patch tested?

Manually tested this change with a case where the `coordinator` is null to make sure `ShuffleExchange.nodeName` doesn't throw a `MatchError` any more.

Author: Kris Mok <kris.mok@databricks.com>

Closes #18095 from rednaxelafx/shuffleexchange-nodename.

sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/ShuffleExchange.scala

@@ -40,6 +40,9 @@ case class ShuffleExchange(
     child: SparkPlan,
     @transient coordinator: Option[ExchangeCoordinator]) extends Exchange {
 
+  // NOTE: coordinator can be null after serialization/deserialization,
+  //       e.g. it can be null on the Executor side
+
   override lazy val metrics = Map(
     "dataSize" -> SQLMetrics.createSizeMetric(sparkContext, "data size"))
 
@@ -47,7 +50,7 @@ case class ShuffleExchange(
     val extraInfo = coordinator match {
       case Some(exchangeCoordinator) =>
         s"(coordinator id: ${System.identityHashCode(exchangeCoordinator)})"
-      case None => ""
+      case _ => ""
     }
 
     val simpleNodeName = "Exchange"
@@ -70,7 +73,7 @@ case class ShuffleExchange(
     // the plan.
     coordinator match {
       case Some(exchangeCoordinator) => exchangeCoordinator.registerExchange(this)
-      case None =>
+      case _ =>
     }
   }
 
@@ -117,7 +120,7 @@ case class ShuffleExchange(
           val shuffleRDD = exchangeCoordinator.postShuffleRDD(this)
           assert(shuffleRDD.partitions.length == newPartitioning.numPartitions)
           shuffleRDD
-        case None =>
+        case _ =>
           val shuffleDependency = prepareShuffleDependency()
           preparePostShuffleRDD(shuffleDependency)
       }