Libxml upgrade

Creating a generic interface between Tritium and Gokogiri (https://moovweb.atlassian.net/browse/PFST-49)
there are 2 options I see here:
1) have this explicitly return return nil
2) make this function have the same style as the similar one on line 70

Either way, they should both use the same method (meaning if you go with option 1, you'll need to tweak the function on line 70).