Updated saml2 slo config so url is used if no repsonse url

Updated config to change empty string to null since the empty string was
hitting an isset check which caused an empty string to be used instead
of the slo url as a backup option.

Closes #2002

app/Config/saml2.php

@@ -101,7 +101,7 @@ return [
                 'url' => env('SAML2_IDP_SLO', null),
                 // URL location of the IdP where the SP will send the SLO Response (ResponseLocation)
                 // if not set, url for the SLO Request will be used
-                'responseUrl' => '',
+                'responseUrl' => null,
                 // SAML protocol binding to be used when returning the <Response>
                 // message.  Onelogin Toolkit supports for this endpoint the
                 // HTTP-Redirect binding only