pause the server if no player is connected

Hello

There is no way to officially pause the sever while it's up and running. You can shut the server down when no one is using it if you'd like.

Not sure why you even would want to 'pause'. The game does go into stasis. ARK enable this a long time ago to free up resources. If you play PVE, you dont have to worry about others attacking. And if you are not in the area for a length of time, the area goes into stasis. You can read more about stasis at the ARK wiki.