[VadeRetro]

OK, I get the difference between a data field being specified as zero and simply omitted. Would you say that this equates to the difference between "The energy of the universe is zero" and "'The energy of the universe' does not exist?" (The latter would presumably be true because the universe does not exist.)

[Alamo-Girl]

Yes! One minor clarification - a field which is null is not simply omitted. The field always exists, and thus the possibility of a value - but in the null instance, a value for the field (including zero) it does not exist.