Change optional parameters back to optional.

Author: Potherca

src/Exception.php

@@ -4,7 +4,7 @@
 
 class Exception extends \Exception
 {
-    public static function create(string $error, array $context, ?\Exception $previous): Exception
+    public static function create(string $error, array $context, ?\Exception $previous = null): Exception
     {
         return new self(vsprintf($error, $context), 0, $previous);
     }

src/Server.php

@@ -96,7 +96,7 @@ final public function setNotifications(SolidNotificationsInterface $notification
     //////////////////////////////// PUBLIC API \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
 
     // @TODO: The Graph should be injected by the caller
-    final public function __construct(Filesystem $filesystem, Response $response, ?Graph $graph)
+    final public function __construct(Filesystem $filesystem, Response $response, ?Graph $graph = null)
     {
         $this->basePath = '';
         $this->baseUrl = '';