OldValue=Null you don't know the Old value so the Difference is Unknown (Null)
OldValue=0 You know the Old value, it is 0, so the Difference is Infinity (Null)
If its important to distinguish between these two, simply store the Old value as PreviousValue and display its somewhere.
Edit based on the comment below:
I would just ask him what he wants do in the following situations and show him examples of each of these. Then and ask him "What value do you want to display for these?"
Do not in any way make this a computer issue.. It's a business question.