allow null application slug/name from related field

[ekoyle]

Details

Workaround for required=False not being honored by django-rest-framework and/or drf_spectacular. Fixes goauthentik#9787 (for python bindings, at least).

This at least allows responses to be validated, however schema.yml still lists these response fields as required.

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Checklist

• [ ] Local tests pass (ak test authentik/)
• [x] The code has been formatted (make lint-fix)

If an API change has been made

• [x] The API schema has been updated (make gen-build)

If changes to the frontend have been made

• [ ] The code has been formatted (make web)

If applicable

• [ ] The documentation has been updated
• [ ] The documentation has been formatted (make website)