set empty Gdx.* fields and made get[Component]() methods work in headless backend

`HeadlessApplication#get[Input/Graphics/Audio]()` returned null even though `HeadlessApplication` had corresponding mock implementations.  
Correct me if I'm wrong, but I guess this was because the getters were forgotten when those mock implementations were added.

I don't think they returned null intentionally because the `Gdx.[audio/graphics]` fields were set (`Gdx.input` wasn't though, so it seems it was forgotten as well).