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>

src/bin/pgbench/pgbench.c

@@ -1898,6 +1898,7 @@ top:
 			{
 				case PGRES_COMMAND_OK:
 				case PGRES_TUPLES_OK:
+				case PGRES_EMPTY_QUERY:
 					break;		/* OK */
 				default:
 					fprintf(stderr, "client %d aborted in state %d: %s",