Allow having no pre-authentication flow for SAML Source

**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:

Start at the SAML Source settings and trace how the required pre-authentication flow is presented and executed. Done means a SAML Source can omit pre-authentication or automatically skip an empty flow, without showing the unnecessary continue screen.

Written by the indexing model from the issue text.