Use Node#insertBefore in all cases.

This is a rather trivial change; it is essentially one of style, not substance, so I can understand if you don’t want to merge it.

From the commit message, which just paraphrases [MDN](https://developer.mozilla.org/en-US/docs/Web/API/Node.insertBefore):

> When Node#insertBefore’s second argument is null or undefined, the
> child node is inserted at the end of the parent’s list of child
> nodes. Internet Explorer throws an error when that second argument
> is undefined, but not null.