[SPARK-26287][CORE] Don't need to create an empty spill file when memory has no records

## What changes were proposed in this pull request?
 If there are no records in memory, then we don't need to create an empty temp spill file.

## How was this patch tested?
Existing tests

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Closes apache#23225 from wangjiaochun/ShufflSorter.

Authored-by: 10087686 <wang.jiaochun@zte.com.cn>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: wangjiaochun

core/src/main/java/org/apache/spark/shuffle/sort/ShuffleExternalSorter.java

@@ -145,6 +145,15 @@ final class ShuffleExternalSorter extends MemoryConsumer {
    */
   private void writeSortedFile(boolean isLastFile) {
 
+    // This call performs the actual sort.
+    final ShuffleInMemorySorter.ShuffleSorterIterator sortedRecords =
+      inMemSorter.getSortedIterator();
+
+    // If there are no sorted records, so we don't need to create an empty spill file.
+    if (!sortedRecords.hasNext()) {
+      return;
+    }
+
     final ShuffleWriteMetricsReporter writeMetricsToUse;
 
     if (isLastFile) {
@@ -157,10 +166,6 @@ private void writeSortedFile(boolean isLastFile) {
       writeMetricsToUse = new ShuffleWriteMetrics();
     }
 
-    // This call performs the actual sort.
-    final ShuffleInMemorySorter.ShuffleSorterIterator sortedRecords =
-      inMemSorter.getSortedIterator();
-
     // Small writes to DiskBlockObjectWriter will be fairly inefficient. Since there doesn't seem to
     // be an API to directly transfer bytes from managed memory to the disk writer, we buffer
     // data through a byte array. This array does not need to be large enough to hold a single

core/src/test/java/org/apache/spark/shuffle/sort/UnsafeShuffleWriterSuite.java

@@ -235,6 +235,7 @@ public void writeEmptyIterator() throws Exception {
     final Option<MapStatus> mapStatus = writer.stop(true);
     assertTrue(mapStatus.isDefined());
     assertTrue(mergedOutputFile.exists());
+    assertEquals(0, spillFilesCreated.size());
     assertArrayEquals(new long[NUM_PARTITITONS], partitionSizesInMergedFile);
     assertEquals(0, taskMetrics.shuffleWriteMetrics().recordsWritten());
     assertEquals(0, taskMetrics.shuffleWriteMetrics().bytesWritten());