What would the XML look like in case of a 'null association'? I don't know of a way in which the ITS supports this.
<class> <association nullFlavor='NI' xsi:nil='true'/> ... </class>
There is no concept of a null association, but there is the concept of an association to a null object. So the XML would look like this:
<class> <association nullFlavor='NI'/> </class>
So, there is no ...: if the object is null there can be no elements (though attributes such as classCode and moodCode could be appropriate) And xsi:nil would not be appropriate, though this is a matter of interpretation
Where there is a problem is the difference between a list of null associations and a null list of associations.
The interpretation that the XML SIG took when asked about this in the past was that xsi:nil could and should be used for null associations -- and there was pushback that required nillable to be put back into the schemas -- it had been removed for some releases in 2004/5 (from memory - dates need checking). I think that the question may also be addressing whether associative classes can be null -- and that is an MnM question that it would be useful to get clarity on. The point that I am making here is that when nullFlavor is used, xsi:nil can also be used, as indicated by xsi:nillable in the schemas.