[FIX] mail: mails fetch

`node.get('class', '')` returns a `Nonetype`
in some cases, and, therefore,
`'o_mail_notification' in node.get('class', '')`
throws a
`TypeError: argument of type 'NoneType' is not iterable`.

Replacing by `(node.get('class') or '')` keep the behavior
that was expected, but will prevent to have a `Nonetype`

opw-656508
opw-656753
opw-653965

addons/mail/models/mail_thread.py

@@ -1239,7 +1239,7 @@ class MailThread(models.AbstractModel):
         root = lxml.html.fromstring(body)
         postprocessed = False
         for node in root.iter():
-            if 'o_mail_notification' in node.get('class', '') or 'o_mail_notification' in node.get('summary', ''):
+            if 'o_mail_notification' in (node.get('class') or '') or 'o_mail_notification' in (node.get('summary') or ''):
                 postprocessed = True
                 node.getparent().remove(node)
         if postprocessed: