allow for empty notification on message

test plan
 - create a ticket via email connector
 - it should work

fixes KNO-385

Change-Id: I4ea4e60ab5818dde0879ea4d4776520a5b36f10a
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/232711
Reviewed-by: Jonathan Featherstone <jfeatherstone@instructure.com>
Reviewed-by: Ben Nelson <bnelson@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

app/models/message.rb

@@ -642,7 +642,7 @@ class Message < ActiveRecord::Base
           notification_message,
           path_type,
           target,
-          self.notification.priority?
+          self.notification&.priority?
         )
       end
       complete_dispatch