Pass correct parameter to session_set_cookie_params

lib/SimpleSAML/SessionHandlerPHP.php

@@ -372,7 +372,7 @@ class SessionHandlerPHP extends SessionHandler
             session_set_cookie_params(
                 $cookieParams['lifetime'],
                 $cookieParams['path'],
-                $cookieParams['domain'],
+                $cookieParams['domain'] ?? '',
                 $cookieParams['secure'],
                 $cookieParams['httponly']
             );