Is NULLIF nullable?

Recently I asked about why a field from the nullable side of an OUTER JOIN 
was causing the JOIN to be inefficient, and was told that it was because 
that field had a CASE statement as part of its definition, and that CASE 
(and by extension COALESCE) were non-nullable constructs.

Is NULLIF nullable, in that sense?

Reason:  I'd like to define a VIEW with a field definition thusly:

'P.O. Box' || NULLIF( po_box, ' '::char ) AS ...

And I would like the field thusly defined to be nullable.

-- Dean