Remove field access from EmptyCallback

[yara-blue]

There is no reason why this: https://github.com/RustAudio/rodio/blob/70c236c7ac495b9ccb82716ffba06cb3fe5fa44c/src/source/empty_callback.rs#L10

is public. Same for the callback. The struct should only be made by the factory function EmptyCallback::new.

[PetrGlad]

Should be done with #678