NPE check on AudioNode read

git-svn-id: https://jmonkeyengine.googlecode.com/svn/trunk@8228 75d07b2b-3a1a-0410-a2c5-0572b91ccdca

engine/src/core/com/jme3/audio/AudioNode.java

@@ -758,7 +758,8 @@ public class AudioNode extends Node {
         
         positional = ic.readBoolean("positional", false);
         
-        data = im.getAssetManager().loadAudio(key);
+        if (key != null)
+            data = im.getAssetManager().loadAudio(key);
     }
 
     @Override