Aggregations top_hits: Fixed inconsistent sorting of the hits

In the reduce logic of the top_hits aggregation if the first shard result to process contained has no results then the merging of all the shard results can go wrong resulting in an incorrect sorted hits.
This bug can only manifest with a sort other than score.

Closes #7697

src/main/java/org/elasticsearch/search/aggregations/metrics/tophits/InternalTopHits.java

@@ -94,20 +94,23 @@ public InternalAggregation reduce(ReduceContext reduceContext) {
         List<InternalAggregation> aggregations = reduceContext.aggregations();
         TopDocs[] shardDocs = new TopDocs[aggregations.size()];
         InternalSearchHits[] shardHits = new InternalSearchHits[aggregations.size()];
+        TopDocs topDocs = this.topDocs;
         for (int i = 0; i < shardDocs.length; i++) {
             InternalTopHits topHitsAgg = (InternalTopHits) aggregations.get(i);
             shardDocs[i] = topHitsAgg.topDocs;
             shardHits[i] = topHitsAgg.searchHits;
+            if (topDocs.scoreDocs.length == 0) {
+                topDocs = topHitsAgg.topDocs;
+            }
+        }
+        final Sort sort;
+        if (topDocs instanceof TopFieldDocs) {
+            sort = new Sort(((TopFieldDocs) topDocs).fields);
+        } else {
+            sort = null;
         }
 
         try {
-            final Sort sort;
-            if (topDocs instanceof TopFieldDocs) {
-                sort = new Sort(((TopFieldDocs) topDocs).fields);
-            } else {
-                sort = null;
-            }
-
             int[] tracker = new int[shardHits.length];
             TopDocs reducedTopDocs = TopDocs.merge(sort, from, size, shardDocs);
             InternalSearchHit[] hits = new InternalSearchHit[reducedTopDocs.scoreDocs.length];