Make ExecutionMode.HashResults handle null value

In Spark 1.6, if a value is null, `getLong` will throw an exception. Before 1.6, it will return 0. With this PR, we will check if the result is null. If it is null, null will be returned instead of 0.

Author: Yin Huai <yhuai@databricks.com>

Closes #41 from yhuai/fixSumHash.

src/main/scala/com/databricks/spark/sql/perf/Benchmark.scala

@@ -607,14 +607,13 @@ abstract class Benchmark(
             case ExecutionMode.HashResults =>
               val columnStr = dataFrame.schema.map(_.name).mkString(",")
               // SELECT SUM(HASH(col1, col2, ...)) FROM (benchmark query)
-              val ret =
+              val row =
                 dataFrame
                   .selectExpr(s"hash($columnStr) as hashValue")
                   .groupBy()
                   .sum("hashValue")
                   .head()
-                  .getLong(0)
-              result = Some(ret)
+              result = if (row.isNullAt(0)) None else Some(row.getLong(0))
           }
         }