DRILL-5344: External sort priority queue copier fails with an empty b…

…atch

Unit tests showed that the “priority queue copier” does not handle an
empty batch. This has not been an issue because code elsewhere in the
sort specifically works around this issue. This fix resolves the issue
at the source to avoid the need for future work-arounds.

closes #778

exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/xsort/managed/PriorityQueueCopierTemplate.java

@@ -57,9 +57,12 @@ public void setup(FragmentContext context, BufferAllocator allocator, VectorAcce
 
     queueSize = 0;
     for (int i = 0; i < size; i++) {
-      vector4.set(i, i, batchGroups.get(i).getNextIndex());
-      siftUp();
-      queueSize++;
+      int index = batchGroups.get(i).getNextIndex();
+      vector4.set(i, i, index);
+      if (index > -1) {
+        siftUp();
+        queueSize++;
+      }
     }
   }