add an empty constructor to Grid mesh (#747)

* add an empty constructor with logical values for serialization purposes. eg. the BinaryImporter has issues instantiating the Grid mesh.

* empty constructor for serialization purposes

jme3-core/src/main/java/com/jme3/scene/debug/Grid.java

@@ -32,9 +32,9 @@
 package com.jme3.scene.debug;
 
 import com.jme3.scene.Mesh;
-import com.jme3.scene.Mesh.Mode;
 import com.jme3.scene.VertexBuffer.Type;
 import com.jme3.util.BufferUtils;
+
 import java.nio.FloatBuffer;
 import java.nio.ShortBuffer;
 
@@ -45,6 +45,9 @@ import java.nio.ShortBuffer;
  */
 public class Grid extends Mesh {
 
+    public Grid() {
+    }
+
     /**
      * Creates a grid debug shape.
      * @param xLines