Allow empty queries in pgbench.

This might have been too much of a foot-gun before 9.6, but with the
new commands-end-at-semicolons parsing rule, the only way to get an
empty query into a script is to explicitly write an extra ";".
So we may as well allow the case.

Fabien Coelho

Patch: <alpine.DEB.2.20.1607090922170.3412@sto>