Promises "if not omitted" language

The DOM spec says that callbacks in `then` should be wrapped and added to
the context object "if not omitted":

> Let fulfillWrapper be a promise wrapper callback for resolver and
fulfillCallback if fulfillCallback is not omitted and a promise callback
for resolver and its fulfill algorithm otherwise

A+ and most promises implementations allow non-function values like `null`
to be ignored, but the "if not omitted" language means that only
`undefined` is ignored. Can it be changed to "if
IsCallable(fulfillCallback)"?

Juan