save lastTxn, not empty txn
save lastTxn, not empty txn

I think this behavior is currently irrelevant, because revising
relies on the in-memory lastTxn of the hostUploader rather than
loading from disk. But better to be safe.

ericflo/Sia,mharkus/Sia,ericflo/Sia,NebulousLabs/Sia,NebulousLabs/Sia,MrStrong/Sia,ChrisSchinnerl/Sia,rivine/rivine,johnathanhowell/Sia,MrStrong/Sia,mnsl/Sia,Mingling94/Sia,mtlynch/Sia,ChrisSchinnerl/Sia,mnsl/Sia,rivine/rivine,mharkus/Sia,mtlynch/Sia