handled empty results in terms aggs (opensearch-project#20451)

Co-authored-by: Aniket Modak <animodak@amazon.com>

Author: animodak7

server/src/main/java/org/opensearch/search/aggregations/bucket/composite/CompositeAggregator.java

@@ -735,6 +735,9 @@ public void collect(int doc, long zeroBucket) throws IOException {
 
     @Override
     public List<InternalAggregation> convert(Map<String, Object[]> shardResult, SearchContext searchContext) {
+        if(shardResult.isEmpty()) {
+            return Collections.singletonList(buildEmptyAggregation());
+        }
         // Generate the composite keys
         List<Comparable<?>> currentCompositeKey = new ArrayList<>(sourceConfigs.length);
         List<CompositeKey> compositeKeys = new ArrayList<>(shardResult.size());

server/src/main/java/org/opensearch/search/aggregations/bucket/terms/AbstractStringTermsAggregator.java

@@ -121,6 +121,9 @@ protected SignificantStringTerms buildEmptySignificantTermsAggregation(long subs
 
     @Override
     public List<InternalAggregation> convert(Map<String, Object[]> shardResult, SearchContext searchContext) {
+        if(shardResult.isEmpty()) {
+            return Collections.singletonList(buildEmptyTermsAggregation());
+        }
         int rowCount = shardResult.get(shardResult.keySet().stream().findFirst().get()).length;
         List<StringTerms.Bucket> buckets = new ArrayList<>(rowCount);
         for (int row = 0; row < rowCount; row++) {

server/src/main/java/org/opensearch/search/aggregations/bucket/terms/NumericTermsAggregator.java

@@ -641,6 +641,9 @@ DoubleTerms buildEmptyResult() {
 
         @Override
         public List<InternalAggregation> convert(Map<String, Object[]> shardResult, SearchContext searchContext) {
+            if(shardResult.isEmpty()) {
+                return Collections.singletonList(buildEmptyAggregation());
+            }
             int rowCount = shardResult.isEmpty() ? 0 : shardResult.get(name).length ;
             List<DoubleTerms.Bucket> buckets = new ArrayList<>(rowCount);
             for (int i = 0; i < rowCount; i++) {
@@ -875,6 +878,9 @@ public void close() {
 
     @Override
     public List<InternalAggregation> convert(Map<String, Object[]> shardResult, SearchContext searchContext) {
+        if(shardResult.isEmpty()) {
+            return Collections.singletonList(buildEmptyAggregation());
+        }
         if (resultStrategy instanceof ShardResultConvertor) {
             return ((ShardResultConvertor) resultStrategy).convert(shardResult, searchContext);
         } else {

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

@@ -63,13 +63,10 @@ public static void convertDFResultGeneric(SearchContext searchContext) {
                     }
                 }).toList();
 
-                InternalAggregations internalAggregations = InternalAggregations.EMPTY;
-
-                if(searchContext.getDFResults().isEmpty() == false) {
-                    internalAggregations = InternalAggregations.from(
+                InternalAggregations internalAggregations = InternalAggregations.from(
                         shardResultConvertors.stream().flatMap(x -> x.convert(dfResult, searchContext).stream()).collect(Collectors.toList())
                     );
-                }
+
                 //LOGGER.info("Converted DF result to internal aggregations: {}", internalAggregations.asList());
                 searchContext.queryResult().aggregations(internalAggregations);
             } catch (IOException e) {