NULL strings... necessary?

Hi,

does any of you have a real example where NULL/NIL strings occur and 
need to be sorted differently than empty strings?

I know one (IMAP SORT HEADER BLAH), but it's a little contrived. Also I 
suspect that if an IMAP client issues SORT SUBJECT, the need to 
differentiate between empty-string subject and no subject is less than 
overwhelming.

I'd like to have a better example to justify the feature.

Arnt