feature #51276 [Notifier] Transport possible to have null (StaffNowa)

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
-------

6120c4bd88 [Notifier] Transport possible to have null

Author: fabpot

Message/NullMessage.php

@@ -40,6 +40,6 @@ public function getOptions(): ?MessageOptionsInterface
 
     public function getTransport(): ?string
     {
-        return $this->decoratedMessage->getTransport() ?? 'null';
+        return $this->decoratedMessage->getTransport() ?? null;
     }
 }

Test/Constraint/NotificationTransportIsEqual.php

@@ -19,9 +19,9 @@
  */
 final class NotificationTransportIsEqual extends Constraint
 {
-    private string $expectedText;
+    private ?string $expectedText;
 
-    public function __construct(string $expectedText)
+    public function __construct(?string $expectedText)
     {
         $this->expectedText = $expectedText;
     }