Do not crash if the string is None

--- I18nHelper.orig.py	2011-04-23 15:19:16.246905842 +0200
+++ I18nHelper.py	2011-04-23 15:23:52.346905845 +0200
@@ -37,6 +37,8 @@
 
 def utf8(str):
   """ Convert a string to unicode using utf8 encoding """
+  if str == None:
+    return "none"
   if isinstance(str, unicode):
       return str
   try: