Fix maxScore check in reduce phase when some scores are NaN (#11267)

Signed-off-by: Jay Deng <jayd0104@gmail.com>

Author: jed326

server/src/internalClusterTest/java/org/opensearch/search/sort/FieldSortIT.java

@@ -268,10 +268,6 @@ public void testIssue6614() throws ExecutionException, InterruptedException {
     }
 
     public void testTrackScores() throws Exception {
-        assumeFalse(
-            "Concurrent search case muted pending fix: https://github.com/opensearch-project/OpenSearch/issues/11189",
-            internalCluster().clusterService().getClusterSettings().get(CLUSTER_CONCURRENT_SEGMENT_SEARCH_SETTING)
-        );
         assertAcked(client().admin().indices().prepareCreate("test").setMapping("svalue", "type=keyword").get());
         ensureGreen();
         index(

server/src/main/java/org/opensearch/search/query/TopDocsCollectorContext.java

@@ -532,7 +532,7 @@ public ReduceableSearchResult reduce(Collection<Collector> collectors) throws IO
                     float score = collector.getMaxScore();
                     if (Float.isNaN(maxScore)) {
                         maxScore = score;
-                    } else {
+                    } else if (!Float.isNaN(score)) {
                         maxScore = Math.max(maxScore, score);
                     }
                 }