[Notifier] Change return type

Author: OskarStark

CHANGELOG.md

@@ -5,6 +5,7 @@ CHANGELOG
 -----
 
  * The component is not marked as `@experimental` anymore
+* [BC BREAK] Changed the return type of `AbstractTransportFactory::getEndpoint()` from `?string` to `string`
 
 5.2.0
 -----

Transport/AbstractTransport.php

@@ -79,7 +79,7 @@ public function send(MessageInterface $message): SentMessage
 
     abstract protected function doSend(MessageInterface $message): SentMessage;
 
-    protected function getEndpoint(): ?string
+    protected function getEndpoint(): string
     {
         return ($this->host ?: $this->getDefaultHost()).($this->port ? ':'.$this->port : '');
     }