Preprocessor_DOM::newPartNodeArray should check that loadXML succeeded

If something manages to get invalid UTF-8 into
Preprocessor_DOM::newPartNodeArray, or anything else that somehow is
invalid XML, it should handle it in the same way that
Preprocessor_DOM::preprocessToObj does rather than having something
further down the line blow up on a PPNode_DOM with a null node.

Bug: 65081
Change-Id: Ic24db455808106e17d49a11e41df33ec170f1206