Alter TopGroups.merge() to not return null if no groups (#16305)

- Add java doc for TopGroups.merge()
- Alter TopGroups.merge() to not return null if no groups, this is to make it consistent with the similar method TopGroups.mergeBlockGroups(), then we don't need to handle the null case in the caller place(coming in next PR which replaces searching with collector with searching with collector manager for TopGroups)

Signed-off-by: Binlong Gao <gbinlong@amazon.com>

Author: gaobinlong

lucene/CHANGES.txt

@@ -292,7 +292,7 @@ New Features
 
 Improvements
 ---------------------
-(No changes)
+* GITHUB#16170: Alter TopGroups.merge() to not return null if no groups. (Binlong Gao)
 
 Optimizations
 ---------------------

lucene/grouping/src/java/org/apache/lucene/search/grouping/TopGroups.java

@@ -119,7 +119,18 @@ static float nonNANmax(float a, float b) {
    * documents of one group do only reside in one shard then the totalGroupCount is exact.
    *
    * <p><b>NOTE</b>: the topDocs in each GroupDocs is actually an instance of TopDocsAndShards
+   *
+   * @param shardGroups list of TopGroups to merge, one per shard; must all share the same group
+   *     sort, doc sort, and top groups.
+   * @param groupSort the {@link Sort} used to sort the groups across shards.
+   * @param docSort the {@link Sort} used to sort documents within each group.
+   * @param docOffset which document to start from within each group (for pagination).
+   * @param docTopN how many top documents to keep within each group.
+   * @param scoreMergeMode how to merge scores across shards; see {@link ScoreMergeMode}.
+   * @return merged TopGroups instance, if there are no groups, returns a TopGroups with an empty
+   *     groups array.
    */
+  @SuppressWarnings("unchecked")
   public static <T> TopGroups<T> merge(
       List<TopGroups<T>> shardGroups,
       Sort groupSort,
@@ -131,7 +142,13 @@ public static <T> TopGroups<T> merge(
     // System.out.println("TopGroups.merge");
 
     if (shardGroups.isEmpty()) {
-      return null;
+      return new TopGroups<>(
+          groupSort.getSort(),
+          docSort.getSort(),
+          0,
+          0,
+          (GroupDocs<T>[]) new GroupDocs<?>[0],
+          Float.NaN);
     }
 
     int totalHitCount = 0;