fix watchtower: sweep_tx must not be None
fix watchtower: sweep_tx must not be None

pooler/electrum-ltc,pooler/electrum-ltc,vialectrum/vialectrum,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,vialectrum/vialectrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,wakiyamap/electrum-mona,spesmilo/electrum,vialectrum/vialectrum,spesmilo/electrum