Fix summary iterator in grouping engine(apache#15658)

This PR fixes the summary iterator to add aggregators in the correct position. The summary iterator is used when dims are not present, therefore the new change is identical to the old one, but seems more correct while reading.

processing/src/main/java/org/apache/druid/query/groupby/GroupingEngine.java

@@ -950,7 +950,10 @@ private static Iterator<ResultRow> summaryRowIterator(GroupByQuery q)
     List<AggregatorFactory> aggSpec = q.getAggregatorSpecs();
     ResultRow resultRow = ResultRow.create(q.getResultRowSizeWithPostAggregators());
     for (int i = 0; i < aggSpec.size(); i++) {
-      resultRow.set(i, aggSpec.get(i).factorize(new AllNullColumnSelectorFactory()).get());
+      resultRow.set(
+          q.getResultRowAggregatorStart() + i,
+          aggSpec.get(i).factorize(new AllNullColumnSelectorFactory()).get()
+      );
     }
     Map<String, Object> map = resultRow.toMap(q);
     for (int i = 0; i < q.getPostAggregatorSpecs().size(); i++) {