Merge pull request #44 from MichaelRoosz/fix-passing-null

Fix a "passing null  to parameter" warning

Archiver.php

@@ -129,7 +129,7 @@ protected function aggregateFromVisits($query, $keyField, $valueField)
 
     protected function cleanCustomVarValue($value)
     {
-        if (strlen($value)) {
+        if ($value !== null && strlen($value)) {
             return $value;
         }
         return self::LABEL_CUSTOM_VALUE_NOT_DEFINED;