When creating classes in EA without the need to define any attribute or operation, the class is rendered without any compartment. This rendering can be an issue for some users, and as such EA lets you change that through the element's display properties in a given diagram.
To do so, right click on the class from your diagram, select "Feature and Compartment Visibility", and enable "structure compartment". You should get the result illustrated as follows:
This is a limited solution since it only works on the current diagram, and EA doesn't let you apply this option onto a selection of classes.