Update changelog

Author: GrahamTheCoder

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 * Optimize away some redundant casts and conversions with strings/chars - [#388](https://github.com/icsharpcode/CodeConverter/issues/388)
 * Improve performance of single file conversion
 * Add AsEnumerable where needed in linq "in" clause - [#544](https://github.com/icsharpcode/CodeConverter/issues/544)
+* Remove redundant empty string coalesce in string comparison - [#500](https://github.com/icsharpcode/CodeConverter/issues/500)
 
 ### C# -> VB