docs: 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)