blockchain: Handle zero-length UTXO backend state.
blockchain: Handle zero-length UTXO backend state.

This updates FetchState for the UTXO backend to return nil if the
serialized state in the database has zero-length.  This is not expected
in normal circumstances, however, due to a bug in upgrade code in a
previous commit, it could end up this way.

decred/dcrd,decred/dcrd