fix empty onchain withdrawals being created every 5 minutes

[djpnewton]

deployed to beryllium-test

https://beryllium-test.zap.me/ln_wallet/list_txs

last empty tx should be

2348331 | 9d13c36abd291b3d230b08f90c339a7ae9165ebcdebf6a4372645792f4f8eefc -- | --