chore: avoid deprecations in PHP 8.4

Author: brotkrueml

src/Exception/AuthenticationException.php

@@ -22,7 +22,7 @@ final class AuthenticationException extends \RuntimeException implements Excepti
     public static function fromFailedAuthentication(
         ClientConfiguration $configuration,
         int $code = 0,
-        \Exception $previous = null,
+        ?\Exception $previous = null,
     ): self {
         $message = \sprintf(
             'Authentication failed for user "%s" on JobRouter base URL "%s"',

src/Exception/HttpException.php

@@ -38,7 +38,7 @@ public static function fromError(
         int $statusCode,
         string $resourceUrl,
         string $error,
-        ClientExceptionInterface $e = null,
+        ?ClientExceptionInterface $e = null,
     ): self {
         $message = \sprintf(
             'Error fetching resource "%s": %s',