[Bug]: Undefined array key "PHP_AUTH_USER" at SystemAddressbook.php

[ChristophWurst]

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

[error] 2023-05-23T07:34:21.000 PHP              Undefined array key "PHP_AUTH_USER" at /var/www/nextcloud/apps/dav/lib/CardDAV/SystemAddressbook.php#237 

Steps to reproduce

1. ?
2. ??
3. ???

Expected behavior

No PHP error

Installation method

None

Nextcloud Server version

master

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

n/a

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response