change a elegant way for SortShuffleWriter.scala

Author: maji2014

core/src/main/scala/org/apache/spark/shuffle/sort/SortShuffleWriter.scala

@@ -50,9 +50,7 @@ private[spark] class SortShuffleWriter[K, V, C](
   /** Write a bunch of records to this task's output */
   override def write(records: Iterator[_ <: Product2[K, V]]): Unit = {
     if (dep.mapSideCombine) {
-      if (dep.aggregator.isEmpty) {
-        throw new IllegalStateException("Aggregator is empty for map-side combine")
-      }
+      require(dep.aggregator.isDefined, "Map-side combine without Aggregator specified!")
       sorter = new ExternalSorter[K, V, C](
         dep.aggregator, Some(dep.partitioner), dep.keyOrdering, dep.serializer)
       sorter.insertAll(records)