Fix SearchResponseMerger#testMergeSearchHits

This commit fixes an edge case in tests where search hits are empty
after the merge but some shards returned hits. This can happen if
the total number of merged hits is less than the provided `from`.

Closes #40553

Author: jimczi

server/src/test/java/org/elasticsearch/action/search/SearchResponseMergerTests.java

@@ -396,7 +396,6 @@ public void testMergeAggs() throws InterruptedException {
         assertEquals(totalCount, bucket.getDocCount());
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/40553")
     public void testMergeSearchHits() throws InterruptedException {
         final long currentRelativeTime = randomLong();
         final SearchTimeProvider timeProvider = new SearchTimeProvider(randomLong(), 0, () -> currentRelativeTime);
@@ -442,6 +441,7 @@ public void testMergeSearchHits() throws InterruptedException {
         float expectedMaxScore = Float.NEGATIVE_INFINITY;
         int numIndices = requestedSize == 0 ? 0 : randomIntBetween(1, requestedSize);
         Iterator<Map.Entry<String, Index[]>> indicesIterator = randomRealisticIndices(numIndices, numResponses).entrySet().iterator();
+        boolean hasHits = false;
         for (int i = 0; i < numResponses; i++) {
             Map.Entry<String, Index[]> entry = indicesIterator.next();
             String clusterAlias = entry.getKey();
@@ -465,6 +465,7 @@ public void testMergeSearchHits() throws InterruptedException {
             float maxScore = scoreSort ? numDocs * scoreFactor : Float.NaN;
             SearchHit[] hits = randomSearchHitArray(numDocs, numResponses, clusterAlias, indices, maxScore, scoreFactor,
                 sortFields, priorityQueue);
+            hasHits |= hits.length > 0;
             expectedMaxScore = Math.max(expectedMaxScore, maxScore);
 
             Object[] collapseValues = null;
@@ -514,7 +515,7 @@ public void testMergeSearchHits() throws InterruptedException {
 
         SearchHits searchHits = searchResponse.getHits();
         // the sort fields and the collapse field are not returned when hits are empty
-        if (searchHits.getHits().length > 0) {
+        if (hasHits) {
             assertArrayEquals(sortFields, searchHits.getSortFields());
             assertEquals(collapseField, searchHits.getCollapseField());
         } else {
@@ -540,7 +541,7 @@ public void testMergeSearchHits() throws InterruptedException {
         SearchHit[] hits = searchHits.getHits();
         if (collapseField != null
                 // the collapse field is not returned when hits are empty
-                && hits.length > 0) {
+                && hasHits) {
             assertEquals(hits.length, searchHits.getCollapseValues().length);
         } else {
             assertNull(searchHits.getCollapseValues());