pgbench: Restore compatibility of --partitions=0

A value of 0 is allowed for this option since its creation, that would
map with the default of having no partitions for pgbench_accounts, but
6f164e6 broke that by enforcing an error.  This commit restores the
original behavior.

Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqGAGobiiHR8nH382HJxqm1mzZs8=3oKPXnXivWoFSZmNA@mail.gmail.com

src/bin/pgbench/pgbench.c

@@ -6865,7 +6865,7 @@ main(int argc, char **argv)
 				break;
 			case 11:			/* partitions */
 				initialization_option_set = true;
-				if (!option_parse_int(optarg, "--partitions", 1, INT_MAX,
+				if (!option_parse_int(optarg, "--partitions", 0, INT_MAX,
 									  &partitions))
 					exit(1);
 				break;