Provide special value null for Lua
The may purpose is provide way to use same NULL value to
separate modules. E.g. this value already uses cjson, ODBC and nginx.
So if you want to write function that can produce value that can be
encoded e.g. with cjson library without any changes then you can use
this library.
local null = require "null".null
-- cjson support this value
str = cjson.encode{key = null}
-- ODBC library also support this
stm = cnn:prepare(sql)
stm:bind(1, null)