Add default value for properties
Add default value for properties

Co-authored-by: Gabby Getz <c6b34fc287a49e37cf50da174e0639e63b3e472e@cesium.com>
CesiumGS/cesium,CesiumGS/cesium,CesiumGS/cesium,CesiumGS/cesium,CesiumGS/cesium