Fix DOMText::split to not error on null object pointer

It tried to add an orphan even when no DomDocument was set. Also porting bug 52656.

Also add an alias to DOMText::isWhitespaceinElementContent
It was renamed to isElementContentWhitespace in the current wc3 recommendation
See http://www.w3.org/TR/DOM-Level-3-Core/core.html#Text3-isElementContentWhitespace