[FIX] mail: avoid cache miss when opening the template previewer

Sometimes the previewer crashes in rendering with cache-based records, hence
trying to check directly the id of the related reference record.

Task-2710804 (Mail: Clean MailThread Posting API)

Part-of: odoo/odoo#99482

addons/mail/wizard/mail_template_preview.py

@@ -72,7 +72,7 @@ class MailTemplatePreview(models.TransientModel):
         copy_depends_values = {'lang': self.lang}
         mail_template = self.mail_template_id.with_context(lang=self.lang)
         try:
-            if not self.resource_ref:
+            if not self.resource_ref.id:
                 self._set_mail_attributes()
             else:
                 copy_depends_values['resource_ref'] = '%s,%s' % (self.resource_ref._name, self.resource_ref.id)