Allow having no pre-authentication flow for SAML Source

[adamskrz]

Is your feature request related to a problem? Please describe.
The SAML Source settings require setting a "Pre-authentication flow". The default for this is a blank flow which doesn't do anything as far as I can tell, other than add an extra screen where users press "continue" - in fact, it seems like its existence causes #7752.

Describe the solution you'd like
There should be an option to not have a pre-auth flow, or to have a flow which automatically skips through if it has no stages.

Describe alternatives you've considered
Add some sort of direct redirect in a stage in the pre-auth flow so the user doesn't have to press continue

Additional context
The unnecessary pre-auth flow screen: