\OneLogin_Saml2_LogoutResponse::$_error can also be null

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

Author: LukasReschke

lib/Saml2/LogoutResponse.php

@@ -32,7 +32,7 @@ class OneLogin_Saml2_LogoutResponse
 
     /**
     * After execute a validation process, if it fails, this var contains the cause
-    * @var string
+    * @var string|null
     */
     private $_error;