Prevent possible exceptions during load in odf.ListStyleToCss

* odfUtils.parseLength can return null, so be prepared for that in some way

* attribute "fo:margin-left" could not be set for <style:list-level-label-alignment>
  as it could be defined by the paragraph style (cmp. TODO above the line)
  so for now just use "0px" to not break other things
Default to "left" for unset fo:text-align in <style:list-level-properties>
Add odf.ListStyleToCssTests
Merge pull request #532 from kossebau/preventCrashWithListStyles

Prevent possible exceptions during load in odf.ListStyleToCss