shell: Go straight to sleep if the sceensaver isn't configured

If the screensaver path is not set, nothing would set the compositor
state to sleeping. In that case, outpus DPMS state was never set to
off.

src/shell.c

@@ -2167,8 +2167,10 @@ launch_screensaver(struct desktop_shell *shell)
 	if (shell->screensaver.binding)
 		return;
 
-	if (!shell->screensaver.path)
+	if (!shell->screensaver.path) {
+		weston_compositor_sleep(shell->compositor);
 		return;
+	}
 
 	if (shell->screensaver.process.pid != 0) {
 		weston_log("old screensaver still running\n");