[FIX] mail : traceback on Marketing Automation Mailings test send

Issue: When test sending a mail in Marketing Automation Mailings, there
is a traceback because we try to set the state of the mail to done,
even though there is no done in mail_mail.state

Steps to reproduce :
 1) Install Marketing Automation
 2) Create/select a campaign
 3) Access the templates of that campaign
 4) Create/select a template
 5) Click Test
 6) Send Sample Email
 -> Traceback

opw-2568210

closes odoo/odoo#77299

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>