Answer
Rather than trying to express "equals X OR is null" in one condition, create two separate events (or two event actions) on the same table/trigger point — one with an "is null" condition and the other with an "equals" condition for the specific string value — so each case is handled by its own straightforward, unambiguous condition instead of relying on NULL-handling inside a combined boolean expression. An alternative approach (though less proven) is computing a genuine boolean-style custom attribute via a PL/SQL function that resolves the OR-with-null logic internally, then writing the event condition against that single true/false attribute instead.