[Bug]: Federated calendar without calendar-color property triggers PHP warning in FederatedCalendarImpl

Bug description

When an incoming federated calendar is rendered and the federated calendar info does not contain the Apple iCal calendar-color property, Nextcloud logs a PHP warning instead of handling the missing value gracefully.

Observed warning:

{
  "app": "PHP",
  "message": "Undefined array key \"{http://apple.com/ns/ical/}calendar-color\" at /var/www/nextcloud/apps/dav/lib/CalDAV/Federation/FederatedCalendarImpl.php#39"
}

Steps to reproduce

1. Set up two federated Nextcloud instances.
2. Share a calendar from instance A to a user on instance B.
3. Open the dashboard or another view on instance B that loads the federated calendar metadata.
4. Check nextcloud.log on instance B.

Expected behavior

Missing calendar-color data should be handled gracefully, for example by returning null or a default/fallback color without logging a PHP warning.

Actual behavior

A PHP warning is logged because FederatedCalendarImpl reads the '{http://apple.com/ns/ical/}calendar-color' array key unconditionally.

Environment

• Nextcloud Server: 33.0.2.2
• Calendar app: 6.2.2
• PHP: 8.4
• OS: Debian 13
• Web server: Nginx

Additional information

From local debugging on the affected instance, the federated calendar data passed into FederatedCalendarImpl can exist without the '{http://apple.com/ns/ical/}calendar-color' key, but FederatedCalendarImpl::getDisplayColor() accesses that key directly.

A local null-check workaround stops the warning, but this should be handled upstream.

Start in apps/dav/lib/CalDAV/Federation/FederatedCalendarImpl.php, focusing on FederatedCalendarImpl::getDisplayColor() and its direct access to the calendar-color key. Verify the missing property case first, then confirm the completed change handles it without a PHP warning while preserving a null or fallback color.