>I believe that code as you have it would end up logging as a change
>every case where OLD is NULL and NEW is NULL (i.e., no change at all).

No, it wouldn't. Comparisons with NULL evaluates as follows:

NULL = 1 false
NULL = NULL false
NULL <> 1 false
NULL <> NULL false

NULL is a state meaning that you don't know and doesn't match anything.

Set