Imported orphan documents get XWiki.WebHome as their parent

I noticed that during the import all documents that don't have a parent set get a default value of XWiki.WebHome. This is a change introduced by the new model. Since in the XML the parent is empty, and setParent now uses a document reference, parsing the empty reference results in XWiki.WebHome as the target document.

Setting an empty parent from the web editor works, since there's a special check for empty in setParent(String).