Fix a bug where CI_Session_memcached_driver doesn't write empty sessions

Related: #3919
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index cd18599..e9a265f 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -29,6 +29,7 @@
 -  Fixed a bug (#4424) - :doc:`Session Library <libraries/sessions>` triggered a PHP warning when writing a newly created session with the 'redis' driver.
 -  Fixed a bug (#4437) - :doc:`Inflector Helper <helpers/inflector_helper>` function :php:func:`humanize()` didn't escape its ``$separator`` parameter while using it in a regular expression.
 -  Fixed a bug where :doc:`Session Library <libraries/sessions>` didn't properly handle its locks' statuses with the 'memcached' driver.
+-  Fixed a bug where :doc:`Session Library <libraries/sessions>` triggered a PHP warning when writing a newly created session with the 'memcached' driver.
 
 Version 3.0.4
 =============