[FIX] mail: fix error while previewing a template without record

To reproduce the problem:
- Install project
- Remove all tasks
- Go to settings -> Technical -> Email Templates
- Select the template "Project: Task Rating Request"
- Click on Preview
Without the fix, a stack trace is displayed. With the fix, the preview is
displayed with the message "No record for this model" for the field "Test
record".

Technical note: the error happens because error_msg was not assigned in the
compute method of that field when the reference to the record is missing or
incomplete.

Task-3196081

closes odoo/odoo#116431

X-original-commit: 1f296539210a07e0b1aed0d9d3cd736c5cb33375
Signed-off-by: Dufays Pierre-Yves (pydu) <pydu@odoo.com>
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mail/wizard/mail_template_preview.py

@@ -74,6 +74,7 @@ class MailTemplatePreview(models.TransientModel):
         mail_template = self.mail_template_id.with_context(lang=self.lang)
         if not self.resource_ref or not self.resource_ref.id:
             self._set_mail_attributes()
+            self.error_msg = False
         else:
             try:
                 mail_values = mail_template.with_context(template_preview_lang=self.lang)._generate_template(