Remove empty results before merging

We addressed the empty top docs issue with elastic#126385 specifically for scenarios where
empty top docs don't go through the wire. Yet they may be serialized from data node
back to the coord node, in which case they will no longer be equal to Lucene#EMPTY_TOP_DOCS.

This commit expands the existing filtering of empty top docs to include also those that
did go through serialization.

Closes elastic#126742

Author: javanna

server/src/main/java/org/elasticsearch/action/search/SearchPhaseController.java

@@ -54,6 +54,7 @@
 import org.elasticsearch.search.suggest.completion.CompletionSuggestion;
 
 import java.io.IOException;
+import java.lang.reflect.Array;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -150,11 +151,11 @@ static TopDocs mergeTopDocs(List<TopDocs> results, int topN, int from) {
             return topDocs;
         } else if (topDocs instanceof TopFieldGroups firstTopDocs) {
             final Sort sort = new Sort(firstTopDocs.fields);
-            final TopFieldGroups[] shardTopDocs = results.stream().filter(td -> td != Lucene.EMPTY_TOP_DOCS).toArray(TopFieldGroups[]::new);
+            final TopFieldGroups[] shardTopDocs = removeEmptyResults(results).toArray(TopFieldGroups[]::new);
             mergedTopDocs = TopFieldGroups.merge(sort, from, topN, shardTopDocs, false);
         } else if (topDocs instanceof TopFieldDocs firstTopDocs) {
             final Sort sort = checkSameSortTypes(results, firstTopDocs.fields);
-            final TopFieldDocs[] shardTopDocs = results.stream().filter((td -> td != Lucene.EMPTY_TOP_DOCS)).toArray(TopFieldDocs[]::new);
+            final TopFieldDocs[] shardTopDocs = removeEmptyResults(results).toArray(TopFieldDocs[]::new);
             mergedTopDocs = TopDocs.merge(sort, from, topN, shardTopDocs);
         } else {
             final TopDocs[] shardTopDocs = results.toArray(new TopDocs[numShards]);
@@ -163,6 +164,18 @@ static TopDocs mergeTopDocs(List<TopDocs> results, int topN, int from) {
         return mergedTopDocs;
     }
 
+    private static <T extends TopDocs> List<T> removeEmptyResults(List<T> results) {
+        List<T> nonEmptyResults = new ArrayList<>();
+        for (T result : results) {
+            if (result.totalHits.value() > 0 || result.totalHits.relation() != Relation.EQUAL_TO) {
+                nonEmptyResults.add(result);
+            } else {
+                assert result.scoreDocs.length == 0;
+            }
+        }
+        return nonEmptyResults;
+    }
+
     private static Sort checkSameSortTypes(Collection<TopDocs> results, SortField[] firstSortFields) {
         Sort sort = new Sort(firstSortFields);
         if (results.size() < 2) return sort;