default Collection<String> getDescendantTickets() {
        return new HashSet<>();
    }
Gets descendant tickets. These are generally ticket ids
whose life-line is separate from the TGT until and unless
the TGT goes away entirely. Things such as OAuth access tokens
are a good example of such linked tickets.

@return the descendant tickets
@since 5.1
api/cas-server-core-api-ticket/src/main/java/org/apereo/cas/ticket/TicketGrantingTicket.java