Return '' instead of 'null'

This commit changes the behavior of getURLParameter(name) to return an
empty string when the parameter is not set or has empty value. Before it
returned 'null' as string.

core/js/js.js

@@ -1723,7 +1723,7 @@ function formatDate(timestamp){
  */
 function getURLParameter(name) {
 	return decodeURI(
-			(RegExp(name + '=' + '(.+?)(&|$)').exec(location.search) || [, null])[1]
+			(RegExp(name + '=' + '(.+?)(&|$)').exec(location.search) || [, ''])[1]
 			);
 }