Add test for empty iterator in UnsafeSorter

Author: JoshRosen

core/src/test/java/org/apache/spark/unsafe/sort/UnsafeSorterSuite.java

@@ -22,6 +22,7 @@
 
 import org.junit.Assert;
 import org.junit.Test;
+import static org.mockito.Mockito.*;
 
 import org.apache.spark.HashPartitioner;
 import org.apache.spark.unsafe.PlatformDependent;
@@ -43,6 +44,18 @@ private static String getStringFromDataPage(Object baseObject, long baseOffset)
     return new String(strBytes);
   }
 
+  @Test
+  public void testSortingEmptyInput() {
+    final UnsafeSorter sorter = new UnsafeSorter(
+      new TaskMemoryManager(new ExecutorMemoryManager(MemoryAllocator.HEAP)),
+      mock(UnsafeSorter.RecordComparator.class),
+      mock(UnsafeSorter.PrefixComputer.class),
+      mock(UnsafeSorter.PrefixComparator.class),
+      100);
+    final Iterator<UnsafeSorter.KeyPointerAndPrefix> iter = sorter.getSortedIterator();
+    assert(!iter.hasNext());
+  }
+
   /**
    * Tests the type of sorting that's used in the non-combiner path of sort-based shuffle.
    */