Add Changelog entry for CI job variables config fix

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.9.0 (unreleased)
+  - Fix error when CI job variables key specified but not defined
   - Fix pipeline status when there are no builds in pipeline
   - Fix Error 500 when using closes_issues API with an external issue tracker
   - Add more information into RSS feed for issues (Alexander Matyushentsev)