Allow MeetingResponse on an event where the acting user is not listed as attendee

The most common case is when you're inviting a group of people using an ldap group
email address, therefore not listing any members of the group as attendees.

Previous code returned an error status to the client preventing it
to create the event in the Kolab calendar. In case of Outlook it created
a situation where Outlook displayed an event, but Kolab webmail (and other
clients) knew nothing about it.