SQL SERVER - How to interpret empty string as NULL value

When is in varchar field saved NULL value, is situation easy:

select isnull( revision, '< null >' ) from drawing_drawing;

Output is:

----------
< null >

Different situation is when varchar field is only empty. In that case isnull() returns only empty string:

select isnull( revision, '< null >' ) from drawing_drawing;

Output is:

----------

The solution is to use inner function nullif() - ...question: which value is NULL ?

select isnull( nullif( revision, '' ), '< null >' ) from drawing_drawing;

Output is:

----------
< null >