Fix Audio stop() (#25336)

* Fix Audio stop()

Fix stop()

When source is null, the method will throw an Exception

* Update Audio.js

Co-authored-by: mrdoob <info@mrdoob.com>

src/audio/Audio.js

@@ -169,8 +169,13 @@ class Audio extends Object3D {
 
 		this._progress = 0;
 
-		this.source.stop();
-		this.source.onended = null;
+		if ( this.source !== null ) {
+
+			this.source.stop();
+			this.source.onended = null;
+
+		}
+
 		this.isPlaying = false;
 
 		return this;