[SPARK-20164][SQL] AnalysisException not tolerant of null query plan.

## What changes were proposed in this pull request?

The query plan in an `AnalysisException` may be `null` when an `AnalysisException` object is serialized and then deserialized, since `plan` is marked `transient`. Or when someone throws an `AnalysisException` with a null query plan (which should not happen).
`def getMessage` is not tolerant of this and throws a `NullPointerException`, leading to loss of information about the original exception.
The fix is to add a `null` check in `getMessage`.

## How was this patch tested?

- Unit test

Author: Kunal Khamar <kkhamar@outlook.com>

Closes #17486 from kunalkhamar/spark-20164.

sql/catalyst/src/main/scala/org/apache/spark/sql/AnalysisException.scala

@@ -43,7 +43,7 @@ class AnalysisException protected[sql] (
   }
 
   override def getMessage: String = {
-    val planAnnotation = plan.map(p => s";\n$p").getOrElse("")
+    val planAnnotation = Option(plan).flatten.map(p => s";\n$p").getOrElse("")
     getSimpleMessage + planAnnotation
   }
 

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

@@ -2598,4 +2598,12 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
     }
     assert(!jobStarted.get(), "Command should not trigger a Spark job.")
   }
+
+  test("SPARK-20164: AnalysisException should be tolerant to null query plan") {
+    try {
+      throw new AnalysisException("", None, None, plan = null)
+    } catch {
+      case ae: AnalysisException => assert(ae.plan == null && ae.getMessage == ae.getSimpleMessage)
+    }
+  }
 }