to_json(NULL) should to return JSON null instead NULL

Hi

currently JSON support NULL value - but we doesn't convert NULL to JSON
correctly

postgres=# select to_json(NULL::text) IS NULL;
┌──────────┐
│ ?column? │
╞══════════╡
│ t │
└──────────┘
(1 row)

probably should be json "null";

like

postgres=# select json 'null' is null;
┌──────────┐
│ ?column? │
╞══════════╡
│ f │
└──────────┘
(1 row)

?

Regards

see:
http://stackoverflow.com/questions/32283125/why-cant-null-be-converted-to-jsons-null-in-postgresql
http://stackoverflow.com/questions/21120999/representing-null-in-json

Pavel