Correct inconsistent return statements

Author: iMattPro

event/listener.php

@@ -60,7 +60,7 @@ static public function getSubscribedEvents()
 	/**
 	* Load Google Analytics js code
 	*
-	* @return null
+	* @return void
 	* @access public
 	*/
 	public function load_google_analytics()
@@ -72,7 +72,7 @@ public function load_google_analytics()
 	* Add config vars to ACP Board Settings
 	*
 	* @param \phpbb\event\data $event The event object
-	* @return null
+	* @return void
 	* @access public
 	*/
 	public function add_googleanalytics_configs($event)
@@ -109,7 +109,7 @@ public function add_googleanalytics_configs($event)
 	* Validate the Google Analytics ID
 	*
 	* @param \phpbb\event\data $event The event object
-	* @return null
+	* @return void
 	* @access public
 	*/
 	public function validate_googleanalytics_id($event)