[Bug]: No iMIP response sent when accepting an invitation from Outlook through Nextcloud Mail

Bug description

When accepting an invitation from Outlook in Nextcloud Mail, no iMIP response is sent to the organizer, and the status in Outlook isn't updated. However, when the event is deleted, a decline message is sent successfully.

Steps to reproduce

1. Add your email account to Nextcloud Mail.
2. Configure Nextcloud to send iMIP responses through your email account in Nextcloud Mail.
3. Invite yourself to an event using Outlook Calendar.
4. Accept the invitation in Nextcloud Mail.
5. Observe that the status in Outlook isn’t updated and no email is sent.
6. Delete the event in Nextcloud Mail.
7. Observe that a decline message is sent to the organizer, updating the status.

Expected behavior

When accepting an invitation in Nextcloud Mail, an iMIP response should be sent to the organizer, updating the status in Outlook.

Nextcloud Server version

master

Nextcloud Mail version

main

Additional info

Accepting an invitation from Google Calendar triggers a response email to the organizer.

The EventComparisonService.findModified method returns null for the Outlook event, and as a result, no response is sent.

https://github.com/nextcloud/server/blob/1407a1d1d261d244895d0c6e1a7011c086e15148/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php#L132-L134