We do not recommend preheating an empty piece of enameled cast iron on the stovetop as it could damage the enamel coating. Instead, make sure the cookware has water or oil inside it to distribute heat evenly, and be sure to start with low heat while the vessel is getting warmed up.
It is perfectly safe to preheat empty enameled cast iron in the oven since the heat will be more consistently distributed across the entire cooking vessel.