[hooks] fix AnalyzeEmailText: Email.subject can be None

hooks.py

@@ -72,7 +72,7 @@ class AnalyzeEmailText(hook.Operation):
     """check if there are some change state instruction in the mail content"""
 
     def precommit_event(self):
-        text = self.email.subject
+        text = self.email.subject or u''
         for part in self.email.parts_in_order(prefered_mime_type='text/plain'):
             try:
                 text += ' ' + part.actual_content()