Fix crash when no style:list-level-properties is present

Just a little drive-by fix I did on noticing this frequent crash whilst testing ODPs for my `z-index` PR.

Previously, WebODF would simply not load documents which did not come with this optional element in a list style. Documents which contain list styles are potentially affected.

[This document](http://www.kde.org/kdeslides/fosdem2007/2007-02-flaviocastelli-strigi_desktop_integration.odp) is an example that crashes and fails to load; fixed by this PR.
Why `undefined` and not `null`? `getElementsByTagNameNS` returns a `NodeList` (or `HTMLCollection` in most implementations). And accessing out-of-bounds returns `null` in the docs that I saw.