[SPARK-7719] Re-add UnsafeShuffleWriterSuite test that was removed for Java 6 compat

This patch re-adds a test which was removed in 9ebb44f8 due to a Java 6 compatibility issue.  We now use Guava's `Iterators.emptyIterator()` in place of `Collections.emptyIterator()`, which isn't present in all Java 6 versions.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #6298 from JoshRosen/SPARK-7719-fix-java-6-test-code and squashes the following commits:

5c9bd85 [Josh Rosen] Re-add UnsafeShuffleWriterSuite.emptyIterator() test which was removed due to Java 6 issue

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

@@ -26,6 +26,7 @@ import scala.collection.Iterator;
 import scala.reflect.ClassTag;
 import scala.runtime.AbstractFunction1;
 
+import com.google.common.collect.Iterators;
 import com.google.common.collect.HashMultiset;
 import com.google.common.io.ByteStreams;
 import org.junit.After;
@@ -252,6 +253,20 @@ public class UnsafeShuffleWriterSuite {
     createWriter(false).stop(false);
   }
 
+  @Test
+  public void writeEmptyIterator() throws Exception {
+    final UnsafeShuffleWriter<Object, Object> writer = createWriter(true);
+    writer.write(Iterators.<Product2<Object, Object>>emptyIterator());
+    final Option<MapStatus> mapStatus = writer.stop(true);
+    assertTrue(mapStatus.isDefined());
+    assertTrue(mergedOutputFile.exists());
+    assertArrayEquals(new long[NUM_PARTITITONS], partitionSizesInMergedFile);
+    assertEquals(0, taskMetrics.shuffleWriteMetrics().get().shuffleRecordsWritten());
+    assertEquals(0, taskMetrics.shuffleWriteMetrics().get().shuffleBytesWritten());
+    assertEquals(0, taskMetrics.diskBytesSpilled());
+    assertEquals(0, taskMetrics.memoryBytesSpilled());
+  }
+
   @Test
   public void writeWithoutSpilling() throws Exception {
     // In this example, each partition should have exactly one record: