oauth2: Don't show registration_endpoint if config is undefined

[sawadashota]

Related issue

#1448

Proposed changes

Don't show registration_endpoint at /.well-known/openid-configuration if config is undefined.

Checklist

• I have read the contributing guidelines
• 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 [email protected]) 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 necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

[CLAassistant]

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[aeneasr]

Thank you! That makes sense. By the way, I think omitempty works also with empty strings, no need for poimters afaik :)

[sawadashota]

Thanks! Your are right!
https://play.golang.org/p/Typ4LJQ85Bl

[aeneasr]

Thank you!