Problems with JSON null

[John Cowan]

In the current version of guile-json, JSON null is represented internally
as #nil.  Because of the magic behavior of #nil, this causes certain
problems when trying to discriminate between various internal JSON
representations.  For example, the programmer will expect (list? j) to
detect a JSON object and (null? j) to detect an empty JSON object.  But
these type discriminators are not correct, because they will also return #t
on #nil.  So one must write unidiomatic things like (or (pair? j) ((eq? j
'())) instead of (list? j).

I suggest (even though it is yet another breaking change) switching to the
Scheme symbol null.  This is portable, printable and rereadable, and not
subject to the above problems.