[FIX] mail: render False

- Have a fresh V12 with sale_management
- Be in debug mode
- Go to sale
- Open any order
- Click "Send by email"
- Click on the bug (upper left of the modal)
- Edit view form
- Add a context to the "send" button like this one:
  `context="{'custom_layout': 'mail.<incorrect_external_id>'}"`
- Save and Reload the view
- Click "Send by email"
- Click "Send"

A traceback is raised.

This occurs because we call `render` on `False`.

opw-2246758

closes odoo/odoo#50559

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>