If there are no elements it returns an empty list. For reference - here is a small test family that has an object on the Clearance-Space Protection sub category, and one on the Laundry sub category.
ObjectSubCategoryTest.rfa (432 KB)
After some additional testing, it seems the dash in the name is causing the error… Clearance Space Protection works, Clearance-Space Protection does not work 
edit: underscore works though… Could this be related to the issues Revit has when using operators in parameter names?