[SPARK-10192][HOTFIX] Fix NPE in test that was added in #8402

This fixes an NPE introduced in SPARK-10192 / #8402.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #9620 from JoshRosen/SPARK-10192-hotfix.

core/src/test/scala/org/apache/spark/scheduler/DAGSchedulerSuite.scala

@@ -1070,7 +1070,7 @@ class DAGSchedulerSuite
     // Run the first job successfully, which creates one shuffle dependency
 
     val shuffleMapRdd = new MyRDD(sc, 2, Nil)
-    val shuffleDep = new ShuffleDependency(shuffleMapRdd, null)
+    val shuffleDep = new ShuffleDependency(shuffleMapRdd, new HashPartitioner(2))
     val reduceRdd = new MyRDD(sc, 2, List(shuffleDep))
     submit(reduceRdd, Array(0, 1))