fix(user-management): Fix deleting user in hidden group(s)

[ChristophWurst]

E.g. guest_app

• Resolves: #

Summary

1. Set up https://github.com/nextcloud/guests
2. Create a guest user
3. Open user management
4. Delete the user

Master/stable25/stable24: 💥 TypeError because e.groups.find(...) is undefined
Here: warning but 😎

TODO

• Do

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[ChristophWurst]

CI failures look unrelated, especially the S3 ones.