When portal is not installed and `auth_signup.invitation_scope` is "b2c",
visitors can create an account, leading to a blank page. Still, accounts can be
required for several use cases in apps that do not require portal (such as
survey).
We here add a landing page for users that created an account but have no
requested redirections and cannot be redirected to a customer portal either.
auth_signup_uninvited is also updated in model to be consistent with config
data.
Tests are added to check this behavior.
Task-2762102
Part-of: odoo/odoo#85703
