\OneLogin_Saml2_LogoutResponse::$_error can also be null
\OneLogin_Saml2_LogoutResponse::$_error can also be null

Signed-off-by: Lukas Reschke <65adda4326914576405c9e3a62f4904d96573863@statuscode.ch>

stayallive/php-saml,onelogin/php-saml