Symfony version(s) affected: 4.3.0-BETA1

Description
When sending email using a new symfony/mailer component an event \Symfony\Component\Mailer\Event\MessageEvent us dispatched.
Mailer component has two subscribers for this event:
\Symfony\Component\Mailer\EventListener\MessageListener
\Symfony\Component\Mailer\EventListener\EnvelopeListener
but non of them are fired in case of an event.

Command bin/console de:ev also doesn't shows these listeners.

How to reproduce

1. create one simple twig template for html part of email, for example:

# templates/email/hello.html.twig
<p>Hi {{ email.to }}!</p>

2. create a console command and inject Mailer in constructor

    private $mailer;

    public function __construct(MailerInterface $mailer)
    {
        $this->mailer = $mailer;
        parent::__construct();
    }

3. put in execute method something like:

        $email = new TemplatedEmail();
        $email
            ->from('no-reply@domain.com')
            ->to('me@domain.com')
            ->subject('Test email message')
            ->htmlTemplate('email/hello.html.twig')
            ;

        $this->mailer->send($email);

3. get an Exception

In HandleMessageMiddleware.php line 82:
A message must have a text and/or an HTML part.
In Email.php line 427:
A message must have a text and/or an HTML part.

This exception raised because \Symfony\Component\Mailer\EventListener\MessageListener was not fired.

Possible Solution
¯\_(ツ)_/¯

Additional context
symfony/messenger is installed with default configuration
symfony/swiftmailer-bundle is not installed