In a strongly typed language with a boolean type, a comparison to null would require a type change. It's 'outside' the set of the boolean type.
Yes, if it exists (as an element of some set), then it is definitely outside the declared set of values. The main question is how we can formalize it, particularly, by defining the set NULL is a member of. For example, the following general approaches are possible:
o Extend (any or some) set of values with a special element (NULL)
o NULL = 0 (empty set)
o NULL = <> (empty tuple)
o Treat NULL as non-existence which entails having no-op if it is met, for example, 2 + NULL + 3 = 5 (just delete NULL which means skip it).