[SQL] Is NULLIF nullable?

[Dean Gibson (DB Administrator)]

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
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