There is a special constant for missing value in Apple Events, we should use it.
Created attachment 10881 [details]
There's also cUndefined, but that one is never mentioned in documentation I could find, and is actually for internal use: <http://listserv.dartmouth.edu/scripts/wa.exe?A2=ind9911&L=macscrpt&T=0&P=9383>.
The null descriptor is technically in the same category as undefined - we are not supposed to return it AFAICT. However, many applications return it now (apparently, Cocoa Scripting is to blame), and we probably need some way to differentiate JS null from JS undefined anyway. The other cases (UnspecifiedType, GetterSetterType) should probably be made to just return AE errors eventually.
Committed revision 16765.