docs: add the changelog entry for the app config defaults fix

Reinstating the Unreleased heading makes its compare link definition live
again, so point it at v4.3.1 instead of the stale v4.2.0.

Signed-off-by: Thomas Müller <323649642+oc-tmueller@users.noreply.github.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: oc-tmueller

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 
+## [Unreleased]
+
+### Fixed
+
+- [645](https://github.com/owncloud/richdocuments/pull/645) - Give the remaining app config settings an empty-string default, so opening a document no longer logs a PHP deprecation on every open
+
 ## [4.3.1] - 2026-09-22
 
 ### Security
@@ -209,7 +215,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 ## [2.4.0] - 2020-07-30
 
 
-[Unreleased]: https://github.com/owncloud/richdocuments/compare/v4.2.0...master
+[Unreleased]: https://github.com/owncloud/richdocuments/compare/v4.3.1...master
 [4.2.0]: https://github.com/owncloud/richdocuments/compare/v4.1.0...v4.2.0
 [4.1.0]: https://github.com/owncloud/richdocuments/compare/v4.0.0...v4.1.0
 [4.0.0]: https://github.com/owncloud/richdocuments/compare/v3.0.1...v4.0.0