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

X-original-commit: 92aef67c