def hasCallbacks(service, nodeIdentifier):
    """
        Return wether there are callbacks registered for a node.

        @param service: The XMPP entity that holds the node.
        @type service: L{JID<twisted.words.protocols.jabber.jid.JID>}
        @param nodeIdentifier: The identifier of the publish-subscribe node.
        @type nodeIdentifier: C{unicode}.
        @returns: Deferred that fires with a boolean.
        @rtype: L{Deferred<twisted.internet.defer.Deferred>}
        """
