[JIT] Support for the NULL type

[mihaibudiu]

SQL supports the NULL type, which is the type of the NULL literal.
There is no way to represent this in the JIT compiler.
This is useful for example to cast NULL to various other types, as in the following program:
SELECT T.COL4 || NULL FROM T
Perhaps the compiler can eliminate this, but right now it doesn't.

[Kixiron]

I think this should be handled on the compiler's side so that it can appropriately implement null's semantics within the given context