Returns the activity entry this record links to, under activity, in the same shape as GET /activities/{id}. The body is an empty object {} in the cases where GET /activityRecipients/{id} returns 403: when the activity entry no longer exists, or it is filtered out for this caller: a deal activity when the caller has no deal access, a mailbox email in another user's private mailbox, an entry for a deal the caller does not own (without permission to see all deals), or a list subscription that was removed. Returns 404 if no record has the id.