Do not raise error when translation_service.translate returns None.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@6434 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Message.py

@@ -100,12 +100,16 @@ class Message(Persistent):
           self.message = self.message.decode('utf8')
       return self.message
     else:
-      return translation_service.translate(self.domain,
+      translated_message = translation_service.translate(
+                                             self.domain,
                                              self.message,
                                              mapping=self.mapping,
                                              context=context,
-                                           default=self.default
-                                        ).encode('utf8')
+                                             default=self.default)
+      if translated_message is not None:
+        return translated_message.encode('utf8')
+      else:
+        return self.message
 
 InitializeClass(Message)
 allow_class(Message)