Do not migrate log without trigger

The notification and resource columns are required so they can not be updated
with NULL value when there is no trigger linked.

issue10622
review369341002
(grafted from 1b99cee3a0fd48ef324a0e56369aa1735270c494)

--HG--
branch : 6.0

notification.py

@@ -8,6 +8,7 @@ from email.mime.multipart import MIMEMultipart
 from email.mime.nonmultipart import MIMENonMultipart
 from email.utils import getaddresses
 
+from sql import Null
 from sql.operators import Concat
 from genshi.template import TextTemplate
 
@@ -387,7 +388,8 @@ class EmailLog(ResourceAccessMixin, ModelSQL, ModelView):
                 where=trigger.id == table.trigger))
         cursor.execute(*table.update(
                 [table.notification, table.resource],
-                [notification, resource]))
+                [notification, resource],
+                where=table.trigger != Null))
         table_h.not_null_action('trigger', 'remove')
 
     def get_date(self, name):