chore: add changelog entry

Author: alanpoulain

CHANGELOG.md

@@ -8,6 +8,7 @@
 * Doctrine: Add `nulls_always_first` and `nulls_always_last` to `nulls_comparison` in order filter (#4103)
 * MongoDB: `date_immutable` support (#3940)
 * DataProvider: Add `TraversablePaginator` (#3783)
+* OpenAPI: Better nullable handling in JSON Schema / OpenAPI (#3518)
 * Swagger UI: Add `swagger_ui_extra_configuration` to Swagger / OpenAPI configuration (#3731)
 
 ## 2.6.3