Empty 'Authenticated User Redirect' config causes error

Problem/Motivation

If your config doesn't contain a value for Authenticated User Redirect config, you get a PHP error if you revisit the login page. I'd expect it to not error and maybe redirect to the homepage by default. Use case is where users have bookmarked the login page and are likly to use it regardless of their session status.

Steps to reproduce

- As admin, remove the relevant value for the config,
- Log in to the site.
- Go to: /saml/drupal_login/[ IdP ]?returnTo=
- Error!

Proposed resolution

Use a default value of <front> to harden code and prevent errors.

It looks like this has been done to some degree already, with the install config and an old update hook but for whatever reason, they didn't cover my setup. Maybe i installed the module before it was done and the D8 update hook didn't fire on D9, which i was using at the time? Bit of an edge case maybe and easily resolved by just adding a value for the config but I thought i'd report anyway. :-)