Do not crash if the string argument is None, LP: #768363

Index: software-properties-0.80.8/softwareproperties/kde/I18nHelper.py
===================================================================
--- software-properties-0.80.8.orig/softwareproperties/kde/I18nHelper.py	2011-04-24 09:17:34.298552016 +0200
+++ software-properties-0.80.8/softwareproperties/kde/I18nHelper.py	2011-04-24 09:17:44.748552015 +0200
@@ -37,6 +37,8 @@
 
 def utf8(str):
   """ Convert a string to unicode using utf8 encoding """
+  if str is None:
+    return "none"
   if isinstance(str, unicode):
       return str
   try: