[SPARK-4274] [SQL] Fix NPE in printing the details of the query plan

Author: Cheng Hao <[email protected]>

Closes #3139 from chenghao-intel/comparison_test and squashes the following commits:

f5d7146 [Cheng Hao] avoid exception in printing the codegen enabled

(cherry picked from commit c764d0a)
Signed-off-by: Michael Armbrust <[email protected]>

Author: chenghao-intel

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -444,7 +444,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
          |${stringOrError(optimizedPlan)}
          |== Physical Plan ==
          |${stringOrError(executedPlan)}
-         |Code Generation: ${executedPlan.codegenEnabled}
+         |Code Generation: ${stringOrError(executedPlan.codegenEnabled)}
          |== RDD ==
       """.stripMargin.trim
   }