In this commit, we have changed behaviour of SEO dialog. Now when you
keep title and description fields empty, then the page will use default
title and description. We have added default_title and default_description
to render the preview with default values when title and description
fields are empty in SEO dialog.
Task ID:1949636
closesodoo/odoo#32449
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>