Fixed a bug where attempting to collide with an empty mesh (0 vertices) would throw an exception rather than returning 0 results.

git-svn-id: https://jmonkeyengine.googlecode.com/svn/trunk@10643 75d07b2b-3a1a-0410-a2c5-0572b91ccdca

engine/src/core/com/jme3/scene/Mesh.java

@@ -896,6 +896,10 @@ public class Mesh implements Savable, Cloneable {
                            BoundingVolume worldBound,
                            CollisionResults results){
 
+        if (getVertexCount() == 0) {
+            return 0;
+        }
+        
         if (collisionTree == null){
             createCollisionData();
         }