Test if contact is set before reading it's language

issue10516
review352181002
(grafted from b4aaf747da380f142c8ab2bd1fc686138280c229)

--HG--
branch : 6.0

ir.py

@@ -79,7 +79,7 @@ class EmailTemplate(metaclass=PoolMeta):
         if isinstance(record, Party):
             usage = Transaction().context.get('usage')
             contact = record.contact_mechanism_get('email', usage=usage)
-            if contact.language:
+            if contact and contact.language:
                 language = contact.language
             elif record.lang:
                 language = record.lang