Do not confuse Settings::getIdPSLOResponseUrl() with bad example

[RoSk0]

When IdP configuration has the responseUrl set to an empty string, as it is currently in the example config file, during the single log out flow when LogoutRequest is received, that could lead to the attempt to respond to the IdP using a RelayState value, rather than singleLogoutService.url value, because unlike Settings::checkIdPSettings(), Settings::getIdPSLOResponseUrl() only checks if responseUrl isset() like is it demonstrated here https://3v4l.org/VhEdl .

In my opinion, Settings::checkIdPSettings() should be improved to error on an empty string in the responseUrl value which is implemented in a separate commit.

[RoSk0]

I've checked the failures and both cases appear legitimate to me, if I understand this correctly, and suggest that \OneLogin_Saml_Settings::$idpSingleLogOutUrl should be made nullable.