[FIX] mass_mailing: force to not use a template for the rendering

If the user configures a default `template_id` for
the `mail.compose.message`,
this template was used by the mass mailing to render the email,
and, among others,
it therefore used the `email_to` of this email template,
to render the recipients,
and it was configured empty or something else than `object.id`,
it did not used the `active_ids` passed in the context to the
`mail.compose.message` `create` call to compute the correct
recipients

To reproduce:
 - In developer mode (?debug)
 - Open the full mail compose message
   e.g. in app sales, open a so, hit new message,
   and hit the button to open the full message composer
 - Set the template to the quotation template is not there already
 - In the debug menu of the dialog, View > Set defaults,
 - Click the radio button to all users, and the default to Use template = ...
 - Then, try to send a mass mailing. It will fail because it cannot
   compute the `email_to` values correctly because the `email_to`
   of the quotation template is empty.

opw-1840287

addons/mass_mailing/models/mass_mailing.py

@@ -785,6 +785,7 @@ class MassMailing(osv.Model):
                 'mass_mailing_id': mailing.id,
                 'mailing_list_ids': [(4, l.id) for l in mailing.contact_list_ids],
                 'no_auto_thread': mailing.reply_to_mode != 'thread',
+                'template_id': None,
             }
             if mailing.reply_to_mode == 'email':
                 composer_values['reply_to'] = mailing.reply_to