Since 3eb9680602, the signature of method
`_notify_by_email_prepare_rendering_context` has been changed to provide
a default values to `msg_vals` and some overrides were not adapted
(or have been added afterwards).
No true bug/issue has been found caused by that discrepancy, but for
consistency, this commit makes sure those overrides are adapted to
provide the same API as the parent method.
Fixes #162742
closes odoo/odoo#163418
X-original-commit: d4b31842d6c3e1c5c86d9019a353601914ebf1f7
Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>