[dom] The mutation method macro has undefined behavior when nodes is empty

foolip has just created a new issue for https://github.com/whatwg/dom:

== The mutation method macro has undefined behavior when nodes is 
empty ==
https://dom.spec.whatwg.org/#mutation-method-macro

This doesn't deal with the case where *nodes* contains no node at all.
 This needs to be defined for `ParentNode.prepend()`, 
`ParentNode.append()`, `ChildNode.before()`, `ChildNode.after()` and 
`ChildNode.replaceWith()` called with no arguments.

The only case where there could be any doubt is `replaceWith()`, 
should that do nothing or remove the context object?

See https://github.com/whatwg/dom/issues/44