Fix crash when certain list style elements are not present.

Specifically:

1. No style:list-level-properties is present.
2. No style:list-level-label-alignment child of that is present.
   In this case, it simply falls back to default zero margins and
   bullet widths.

Previously this would simply not load documents which did not come
with this optional element in a list style.

Layout tests and CSS generation tests are added for both cases.
Merge pull request #533 from adityab/list-level-props-optional

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