Replace NullType with NullableType(BottomType())

[MeAmAnUsername]

Summary
Replace the type NullType() with NullableType(BottomType())

Todo

• Add tests?
• Implement
• Update documentation

Reason
It is inelegant that a type can be nullable without being NullableType(_). This also makes it explicitly clear how the type hierarchy works.

Implementation
NullType() with NullableType(BottomType()) should just work.

Related issues

• Similar issue for empty lists: Replace EmptyList with list of bottom type #282

Background information
This is also how it is done in Kotlin, as is described in this blog post