management: Fix regression with the prefix registration using the default certificate

This commit re-introduces a no longer deprecated feature in

    ndn::nfd::Controller::start(ControlParameter,
                                CommandSuccessCallback, CommandFailCallback,
                                IdentityCertificate,
                                time::milliseconds),

which treats IdentityCertificate() (an invalid certificate that has an empty
name) as the default certificate.

Change-Id: I3f147260d649e82c6bd1927ff5998be79a37d0dc
Refs: #2080