[whatwg] When initializing a JS URL object, do we handle the case of 0 argument?

[Shawn Gong]

(Referenced from Adam Barth's document: 
https://docs.google.com/document/edit?id=1r_VTFKApVOaNIkocrg0z-t7lZgzisTuGTXkdzAk4gLU&hl=en)

The incoming arguments plainURL and baseURL are optional. The question is, when 
there is no argument provided, do we return an error or treat it normally, then 
assign the URL later? If so, which interface do we use to assign the URL? href?

Thanks!
Shawn Gong