SSO Config validation causes fatal error before metadata is saved

In an environment with the plugin network enabled, entering the network settings page to provide the SSO IdP Metadata provides a broken settings page, as the `SSO Config validation` field expects the metadata to exist already when viewing the page.

The outcome of this is that it is impossible to save the page, so you are unable to add the metadata to resolve the issue without manually adding the `sso_idp_metadata` meta key and value to the `wp_sitemeta` database.