Add changelog entry for nested kNN NPE fix

Signed-off-by: ved naykude <vnaykude@amazon.com>

Author: naykudev

CHANGELOG.md

@@ -12,6 +12,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Maintenance
 
 ### Bug Fixes
+* Fix NPE in nested kNN search when index contains documents without nested object [#3368](https://github.com/opensearch-project/k-NN/pull/3368)
 * Turn off ACORN for MOS to match default Lucene HNSW behavior [#3346](https://github.com/opensearch-project/k-NN/pull/3346)
 * Preserve mixed-case derived source vector field names and add backward-compatible field resolution for previously lowercased segment metadata [#3313](https://github.com/opensearch-project/k-NN/pull/3313)
 * Fix rescore flag not propagating over transport layer in multi-node clusters [#3343](https://github.com/opensearch-project/k-NN/pull/3343)