Previously storing an empty batch would make the batcher fail to start
as spinning up a restored batch assumes that there's a primary sweep
added already. As there's no point in spinning up such batch we can just
skip over it.
Furthermore we'll ensure that we won't try to ever publish an empty
batch to avoid setting the fee rate too early.
Previously we'd report the fees per sweep as the total sweep cost of a
batch. With this change the reported cost will be the proportional fee
which should be equal for all sweeps except if there's any rounding
difference in which case that is paid by the sweep belonging to the
first input of the batch tx.
