Fix #70029: nodeValue of DOMElement list content of children nodes

Fix #70029: nodeValue of DOMElement list content of children nodes

According to the W3C DOM Core Level 3 specification DOMElement::nodeValue is
supposed to yield null, but currently the DOM extension yield the result of
calling xmlNodeGetContent(), what is the concatenation of the values of the
children of the element. This patch fixes the non standard behavior.