According to documentation, new unassigned variables should have a value of nil, but they have a value of nothing.
Moreover, according to documentation it doesn’t exist a type called nothing.
The simplest example shows a different thing:
[admin@mt] :global kk ; :put [:typeof $kk]
nothing
Related post: http://forum.mikrotik.com/t/variables-behaviour-in-3-10/21604/1