This PR was merged into the 6.4 branch.

Discussion
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[Notifier] Transport possible to have null

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #51236
| License       | MIT

My test example of why I need to have properly transport null

```php
public function testSmsSending(): void
{
    $date = (new \DateTimeImmutable())->format('Y-m-d');

    $stateta = new Stateta();
    $stateta->setDate(new \DateTimeImmutable($date));
    $stateta->setA95(0);
    $stateta->setDk(0);

    $this->entityManager->persist($stateta);
    $this->entityManager->flush();

    $this->commandTester->execute([], [
        'verbosity' => OutputInterface::VERBOSITY_DEBUG
    ]);

    $this->commandTester->assertCommandIsSuccessful();

    $notifierEvent = $this->getNotifierEvent();

    self::assertEquals(null, $notifierEvent->getMessage()->getTransport());
    self::assertEquals("+37061234567", $notifierEvent->getMessage()->getRecipientId());

    self::assertNotificationIsQueued($notifierEvent);
    self::assertNotificationSubjectContains($notifierEvent->getMessage(), 'Company name');
    self::assertNotificationTransportIsEqual($notifierEvent->getMessage(), null);
}
```

Commits
-------

6120c4b [Notifier] Transport possible to have null