If the food you're cooking in the slow cooker doesn't require any water to cook with then yes you can use the slow cooker without water.
An example of food to cook without water is a bake potato in some tin foil.
You can place the potato in some tin foil or aluminum foil and then let it cook in the slow cooker without the water.
So if your food doesn't require water to cook such as beans etc then yes you can use your slow cooker without water or liquid to cook with.