[spec] "name" and "operationCtor" parameter cannot be missing during register()

[xyaoinum]

Now that register() is specified via WebIDL: undefined register(DOMString name, Function operationCtor), the "name" and "operationCtor" cannot be missing in the actual algorithm. We should simply check whether they are default/empty values (https://github.com/WICG/shared-storage/pull/131/files#r1563075565)