Expose AudioSink::empty() (#8145)

# Objective

Exposes `empty()` method for `AudioSink`.
Based on `0.10.0`, should be a non-breaking change.

---

## Changelog

- Expose `empty()` method for `AudioSink`
- Add `AudioSink::empty()` example

---------

Co-authored-by: hank <hank@hank.co.in>
Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_audio/src/sinks.rs

@@ -60,6 +60,9 @@ pub trait AudioSinkPlayback {
     ///
     /// It won't be possible to restart it afterwards.
     fn stop(&self);
+
+    /// Returns true if this sink has no more sounds to play.
+    fn empty(&self) -> bool;
 }
 
 /// Asset controlling the playback of a sound
@@ -130,6 +133,10 @@ impl AudioSinkPlayback for AudioSink {
     fn stop(&self) {
         self.sink.as_ref().unwrap().stop();
     }
+
+    fn empty(&self) -> bool {
+        self.sink.as_ref().unwrap().empty()
+    }
 }
 
 /// Asset controlling the playback of a sound, or the locations of its listener and emitter.
@@ -197,6 +204,10 @@ impl AudioSinkPlayback for SpatialAudioSink {
     fn stop(&self) {
         self.sink.as_ref().unwrap().stop();
     }
+
+    fn empty(&self) -> bool {
+        self.sink.as_ref().unwrap().empty()
+    }
 }
 
 impl SpatialAudioSink {