fix: use null type instead of optional for generated type

[nbaron1]

When dealing with optional data prisma generates the type as null or the desired type.
Currently the generator makes it undefined or the desired type.

For example the prisma generated type for the Post model is currently

export type Post = {
  id: number
  userId: number | null
}

however the generator creates the type as:

export const Post = Type.Object({
  id: Type.Number(),
  userId: Type.Optional(Type.Number()),
});

which is the same as number | undefined

Fixes: #19

[Dan-Shields]

I encountered the same and switching to m1212e/prismabox (a fork of this repo) fixed the issue by adding __nullable__ to its schemas.