[util] stringToArray: if string is null we should return with empty array

Author: connorhu

lib/util/sfToolkit.class.php

@@ -245,6 +245,10 @@ public static function arrayDeepMerge()
      */
     public static function stringToArray($string)
     {
+        if ($string === null) {
+            return [];
+        }
+
         preg_match_all('/
       \s*((?:\w+-)*\w+)     # key                               \\1
       \s*=\s*               # =