Make sure schema generation handles optional properly

[stuartwdouglas]

In general schema generation should rely on the @NotNull annotation to handle optional values during schema generation. If a field / param has this annotation then it is not optional, otherwise it is. There are some exceptions to this such as primitive values which can never be null.

This should align with Kotlins type system, so Kotlin objects will map natively.

At the moment we don't really take this into account.