Add to change log

CHANGELOG.md

@@ -20,6 +20,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Explainability in hybrid query ([#970](https://github.com/opensearch-project/neural-search/pull/970))
 - Support new knn query parameter expand_nested ([#1013](https://github.com/opensearch-project/neural-search/pull/1013))
 - Implement pruning for neural sparse ingestion pipeline and two phase search processor ([#988](https://github.com/opensearch-project/neural-search/pull/988))
+- Allow empty string for field in field map ([#1041](https://github.com/opensearch-project/neural-search/pull/1041))
 ### Bug Fixes
 -  Address inconsistent scoring in hybrid query results ([#998](https://github.com/opensearch-project/neural-search/pull/998))
 ### Infrastructure