Fix null values when indexing in flat_object field (opensearch-project#14069)

Signed-off-by: kkewwei <[email protected]>

Author: kkewwei

CHANGELOG.md

@@ -83,6 +83,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix indexing error when flat_object field is explicitly null ([#15375](https://github.com/opensearch-project/OpenSearch/pull/15375))
 - Fix split response processor not included in allowlist ([#15393](https://github.com/opensearch-project/OpenSearch/pull/15393))
 - Fix unchecked cast in dynamic action map getter ([#15394](https://github.com/opensearch-project/OpenSearch/pull/15394))
+- Fix null values indexed as "null" strings in flat_object field ([#14069](https://github.com/opensearch-project/OpenSearch/pull/14069))
 
 ### Security