Disable SAML SLO request when LL::NG configured as SP and IDP does not support SLO
By default, SAML SP always tries to launch a SLO request on logout, which is compliant with SAML standard. But we can face some SAML IDP that does not support SLO. In this case, we got an error.
We should prevent SLO request if no SLO endpoints are present in metadata.