Scaffold nullable types as "(type | null)"

[smokku]

Nullable types are generated as optional in typescript files. This makes the fields of type | undefined.

When you try to set null as such field, TS compiler refuses to assign null to optional field.

The main problem is that it prevents sending null to backend to erase field value in mutation.

Workaround is to do something like this

store.mutateSetSomething({
  value: null as unknown as undefined,
})

This PR fixes this by generating nullable types as (type | null).

[jesse-savary]

Hey @smokku, I've added your commits into #382 because the development branch is more up to date than main. Please give version 0.17.0 a go and let me know what you think.