changelog: add notes about closing #13666

[skip ci]

CHANGELOG.md

@@ -24,6 +24,7 @@
 * **Postgres**: Fix template variables error [#13692](https://github.com/grafana/grafana/issues/13692), thx [@svenklemm](https://github.com/svenklemm)
 * **Cloudwatch**: Fix service panic because of race conditions [#13674](https://github.com/grafana/grafana/issues/13674), thx [@mtanda](https://github.com/mtanda)
 * **Stackdriver/Cloudwatch**: Allow user to change unit in graph panel if cloudwatch/stackdriver datasource response doesn't include unit [#13718](https://github.com/grafana/grafana/issues/13718), thx [@mtanda](https://github.com/mtanda)
+* **Variables**: Fix text box template variable doesn't work properly without a default value [#13666](https://github.com/grafana/grafana/issues/13666)
 * **LDAP**: Fix super admins can also be admins of orgs [#13710](https://github.com/grafana/grafana/issues/13710), thx [@adrien-f](https://github.com/adrien-f)
 
 # 5.3.1 (2018-10-16)