An enumeration by default has a NULL (empty) value as well. For example when evaluating a enumeration in an exclusive split, you will see it also has the empty value to consider. F.e.:
You can set it by setting an object to empty. If you want it to be selectable rather use a dropdown button instead of radio buttons as radio buttons cannot be unselected.
The first line here is the NULL (empty) value.
Same if in the radio buttons above, there is nothing selected it is an empty value.