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Documentation: reference/reference_lua/box_null

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	Constant box.NULL
	There are some major problems with using Lua nil values in tables. For example: you can't correctly assess the length of a table that is not a sequence. (Learn more about data types in `Lua <https://www.lua.org/manual/5.1/manual.html#2.2>`__ and `LuaJIT <http://luajit.org/ext_ffi_semantics.html>`__.)
	Example:
	tarantool> t = {0, nil, 1, 2, nil} --- ... tarantool> t --- - - 0 - null - 1 - 2 ... tarantool> #t --- - 4 ...
	The console output of ``t`` processes nil values in the middle and at the end of the table differently. This is due to undefined behavior.
	Trying to find the length for sparse arrays in LuaJIT leads to another scenario of `undefined behavior <https://www.lua.org/manual/5.1/manual.html#2.5.5>`_.
	To avoid this problem, use Tarantool's ``box.NULL`` constant instead of nil. ``box.NULL`` is a placeholder for a nil value in tables to preserve a key without a value.
	Using box.NULL
	``box.NULL`` is a value of the `cdata <http://luajit.org/ext_ffi_semantics.html>`__ type representing a NULL pointer. It is similar to ``msgpack.NULL``, ``json.NULL`` and ``yaml.NULL``. So it is some not nil value, even if it is a pointer to NULL.
	Use ``box.NULL`` only with capitalized NULL (``box.null`` is incorrect).
	Technically speaking, ``box.NULL`` equals to ``ffi.cast('void *', 0)``.
	tarantool> t = {0, box.NULL, 1, 2, box.NULL} --- ... tarantool> t --- - - 0 - null # cdata - 1 - 2 - null # cdata ... tarantool> #t --- - 5 ...
	Notice that ``t[2]`` shows the same ``null`` output in both examples. However in this example ``t[2]`` and ``t[5]`` are of the cdata type, while in the previous example their type was nil.
	Avoid using implicit comparisons with nullable values when using ``box.NULL``. Due to `Lua behavior <https://www.lua.org/manual/5.1/manual.html#2.4.4>`_, returning anything except false or nil from a condition expression is considered as true. And, as it was mentioned earlier, ``box.NULL`` is a pointer by design.
	That is why the expression ``box.NULL`` will always be considered true in case it is used as a condition in a comparison. This means that the code
	``if box.NULL then func() end``
	will always execute the function ``func()`` (because the condition ``box.NULL`` will always be neither false nor nil).
	Distinction of nil and box.NULL
	Use the expression ``if x == nil`` to check if the ``x`` is either a nil or a ``box.NULL``.

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