Create null literal singleton

[BenjaminCosman]

See:

https://github.com/UCSD-PL/RefScript/blob/master/tests/todo/null-undef-literals.ts

[panagosg7]

Test is here:
https://github.com/UCSD-PL/refscript/blob/refactor_types/tests/todo/pos/simple/null-undef-literals.ts