fix: hide guests group when searching for principals

[kesselb]

Resolves: #52958

Summary

Follow-up for https://github.com/nextcloud/server/pull/52914

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)