XWikiDocument#setXObject does not work if there is not already an object of the provided class

This method internally use getXObjects(classReference) to get the actually list to add the provided object to except that the list returned by getXObjects(classReference) is just a new ArrayList<BaseObject>() stored nowhere when there is no object of this class...

Same things for:

• setXObject(DocumentReference classReference, int nb, BaseObject object)
• createXObject
• addXObject
• setXObject(int nb, BaseObject object)

I think getXObjects(DocumentReference) changed at some point and broke them since they are all broken because of a missunderstanding on how it's supposed to work: they all expect it to return null when there is no object of this class but it returns a new ArrayList (which makes a lots more sens than null since it should never return null, Collections#EMPTY_LIST would be event better but I have the crappy code is using getXObject(reference).add()...).