fix: fallback to default return url if logout after url is not defined

[zepatrik]

Related issue

When selfservice.flows.logout.after.default_browser_return_url was not set a logout resulted in a fatal crash.

Proposed changes

The fallback now is selfservice.default_browser_return_url

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments