Fix bug when aggregator is used but map-side combine is disabled

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

@@ -55,7 +55,7 @@ private[spark] class SortShuffleWriter[K, V, C](
       new ExternalSorter[K, V, C](
         dep.aggregator, Some(dep.partitioner), dep.keyOrdering, dep.serializer)
     } else if (SortShuffleWriter.shouldBypassMergeSort(
-        SparkEnv.get.conf, dep.partitioner.numPartitions, dep.aggregator, dep.keyOrdering)) {
+        SparkEnv.get.conf, dep.partitioner.numPartitions, aggregator = None, dep.keyOrdering)) {
       // If there are fewer than spark.shuffle.sort.bypassMergeThreshold partitions and we don't
       // need local aggregation and sorting, write numPartitions files directly and just concatenate
       // them at the end. This avoids doing serialization and deserialization twice to merge
@@ -67,7 +67,8 @@ private[spark] class SortShuffleWriter[K, V, C](
       // In this case we pass neither an aggregator nor an ordering to the sorter, because we don't
       // care whether the keys get sorted in each partition; that will be done on the reduce side
       // if the operation being run is sortByKey.
-      new ExternalSorter[K, V, V](None, Some(dep.partitioner), None, dep.serializer)
+      new ExternalSorter[K, V, V](
+        aggregator = None, Some(dep.partitioner), ordering = None, dep.serializer)
     }
     sorter.insertAll(records)