Way to create null descriptor

### Feature description I'm currently sending multiple array in storage buffer to shader. However that array can be empty. but WriteDescriptorSet currently does not allow writing zero range. so I want to use null descriptor in this case to handle it. Disabling whole shader invocation is not an option in my case. My application is ray tracing shader and these arrays contain one type of primitives each. so if array of spheres is empty, it should still run and trace ray using other primitives like plane and cubes. Given the documents of `WriteDescriptorSet::none`, I don't think that could be used for nullDescriptor.