LPS-33626 XML without prolog is valid

Importing currently throws npe due to the following:

1. document.formattedString() -> element.formattedString() in DataHandlers = no more prolog
2. portletDataContext.setImportDataRootElement(rootElement) is not set because it's not recognized as valid xml
3. Element rootElement = portletDataContext.getImportDataRootElement(); is null causing rootElement.element() to throw NPE