dcrpg: check for an empty database via the blocks table only
dcrpg: check for an empty database via the blocks table only

When detecting an empty database, only checking for the "blocks" table
since some of the tables are created by schema upgrades (e.g.
proposal_votes).

this will bump dcrpg from v3.0.0 to v3.0.1

dcrdata/dcrdata,dcrdata/dcrdata,dcrdata/dcrdata,dcrdata/dcrdata,dcrdata/dcrdata