ecore_audio: implement not implemented APIs as empty

the empty handlers will always return 0 / NULL so never indicate
success.

Reviewed-by: Xavi Artigas <xavierartigas@yahoo.es>
Differential Revision: https://phab.enlightenment.org/D7714

src/lib/ecore_audio/ecore_audio_in.eo

@@ -156,6 +156,8 @@ class Ecore.Audio.In extends Ecore.Audio
       @empty .length {set;}
       @empty .preloaded {set; get;}
       @empty .seek;
+      @empty Ecore.Audio.source {set; get;}
+      @empty Ecore.Audio.format {set; get;}
    }
    events {
       in,looped: void; [[Called when an input has looped.]]