\OneLogin_Saml2_Utils::getExpireTime can also return null
\OneLogin_Saml2_Utils::getExpireTime can also return null

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

onelogin/php-saml,stayallive/php-saml