Change minimum max_worker_processes from 1 to 0

Setting it to 0 is probably not useful in practice, but it allows
testing of situations without available background worker slots.

src/backend/utils/misc/guc.c

@@ -2469,7 +2469,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL,
 		},
 		&max_worker_processes,
-		8, 1, MAX_BACKENDS,
+		8, 0, MAX_BACKENDS,
 		check_max_worker_processes, NULL, NULL
 	},