Don't insert blank text node into every tag (yet another IE8 issue)

Actually Mithril calls build() with data undefined for m("tag",{...}) [calls without third argument].
The changing undefined to "" causes build() insert a TextNode into current tag (on line 169).
But in IE8 it isn't allowed to insert a TextNode into any tag.
For example, m("input",{...}) fails here.

With this simple patch undefined is not chaged to "" anymore.
(And no empty TextNode's added to every mentioned tag.)