ObjectField({ required: false, nullable: true }) doesn't actually allow null/undefined values unless cast is overridden

[JPMeehan]

ObjectField#_cast will transform any null or undefined values into {} even if required: false is set. One example of this is the AdditionalTypesField that's used in BaseModule and as of v12, BaseSystem.

I'm good with the defaults, but maybe _cast should be updated to account for this options or a note should be made that downstream developers should also implement their own _cast method.

[aaclayton]

Pushed a fix to ObjectField which allows it to be initialized as undefined in cases where {required: false} or initialized as null in cases where {required: true, nullable: true}, otherwise initialized as an empty object {}.