channeldb: skip sub-buckets during invoice time series migration
channeldb: skip sub-buckets during invoice time series migration

In this commit, we fix an existing bu gin the invoice time series
migration code. Before this commit, the migration would fail as we would
try to migrate an empty invoice. We now detect this case and skip all
empty invoices.

We also add a bit more logging on both the info and trace logging level.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd