fix(oneOf): identify Nullable type returned from oneOf method

[themagickoala]

This PR is incomplete, as the ts-expect-error tests are failing in one-to-one test files for create and update. My intention here is for the return value of oneOf to default to OneOf<ModelType, false>, and to be OneOf<ModelType, true> if you pass { nullable: true }. However, the types I've added seem to not be specific enough, as the default is just OneOf<ModelType, boolean> whatever you set. @kettanaito are you able to provide some guidance? If not, I'll try and come back to this another time!