fix(pg-native): report a missing row count or command as null

[nigrosimone]

pg-native builds rowCount with parseInt(pq.cmdTuples(), 10) and command with pq.cmdStatus().split(' ')[0] without checking that libpq gave anything. A command with no row count, like BEGIN, comes back with rowCount: NaN, and an empty query with command: ''. pg reports null in both cases, and rowCount is typed as number | null, so code that checks rowCount === null behaves differently on the two clients.

The fix returns null when libpq gives an empty string.

The added test runs BEGIN and an empty query and checks both fields: it fails on master and passes here.