Writes <hexdata> as new nulldata output to <rawtx>.

def add_nulldata(self, rawtx, hexdata):
        """Writes <hexdata> as new nulldata output to <rawtx>."""
        tx = deserialize.unsignedtx(rawtx)
        nulldata_txout = deserialize.nulldata_txout(hexdata)
        tx = control.add_nulldata_output(tx, nulldata_txout)
        return serialize.tx(tx)