[FIX] mass_mailing: try to render jinja content when using test wizards

Before when a user clicked on the button "Test", jinja syntax was not taken
into account for the test mail sent. If an error was present clicking on the
button did not raise an error before the cron was executed.

Now, if there is at least one record in the mailing model, the template is
correctly rendered and throws an error if there is a syntax error.

If we don't have any record to render the template, we fallback on the raw
content like before.

PR odoo/odoo#55696
Task ID-2312442

addons/mass_mailing/wizard/mailing_mailing_test.py

@@ -22,16 +22,28 @@ class TestMassMailing(models.TransientModel):
         mailing = self.mass_mailing_id
         test_emails = tools.email_split(self.email_to)
         mass_mail_layout = self.env.ref('mass_mailing.mass_mailing_mail_layout')
+
+        record = self.env[mailing.mailing_model_real].search([], limit=1)
+        body = mailing._prepend_preview(mailing.body_html, mailing.preview)
+        subject = mailing.subject
+
+        # If there is atleast 1 record for the model used in this mailing, then we use this one to render the template
+        # Downside: Jinja syntax is only tested when there is atleast one record of the mailing's model
+        if record:
+            # Returns a proper error if there is a syntax error with jinja
+            body = self.env['mail.render.mixin']._render_template(body, mailing.mailing_model_real, record.ids, post_process=True)[record.id]
+            subject = self.env['mail.render.mixin']._render_template(subject, mailing.mailing_model_real, record.ids)[record.id]
+
+        # Convert links in absolute URLs before the application of the shortener
+        body = self.env['mail.render.mixin']._replace_local_links(body)
+        body = tools.html_sanitize(body, sanitize_attributes=True, sanitize_style=True)
+
         for test_mail in test_emails:
-            # Convert links in absolute URLs before the application of the shortener
-            body = mailing._prepend_preview(mailing.body_html, mailing.preview)
-            body = self.env['mail.render.mixin']._replace_local_links(body)
-            body = tools.html_sanitize(body, sanitize_attributes=True, sanitize_style=True)
             mail_values = {
                 'email_from': mailing.email_from,
                 'reply_to': mailing.reply_to,
                 'email_to': test_mail,
-                'subject': mailing.subject,
+                'subject': subject,
                 'body_html': mass_mail_layout._render({'body': body}, engine='ir.qweb', minimal_qcontext=True),
                 'notification': True,
                 'mailing_id': mailing.id,