Warnings in MR sync when user not found

If the representing-entry-if of an meeting attendee is not found anymore during sync there are warnings like:

[31937] [WARN] [tschoppe] /usr/share/z-push/backend/kopano/mapiprovider.php:266 Undefined offset: 4260098 (8)

It's not possible to fix the issue, but we could suppress the warning.
Reported here: https://forum.kopano.io/post/242

the line producing this warn is:

if (isset($row[PR_RECIPIENT_TRACKSTATUS]) && $messageprops[$appointmentprops["representingentryid"]] == $storeprops[PR_MAILBOX_OWNER_ENTRYID])
    $attendee->attendeestatus = $row[PR_RECIPIENT_TRACKSTATUS];

We have to check if the representingentryid is available in the messageprops before accessing it.