Fixed ArcRotateCamera target issue

src/Cameras/babylon.arcRotateCamera.js

@@ -88,7 +88,7 @@ var BABYLON;
                 _this._viewMatrix.m[13] += _this.targetScreenOffset.y;
                 _this._collisionTriggered = false;
             };
-            if (!this.target) {
+            if (!target) {
                 this.target = BABYLON.Vector3.Zero();
             }
             else {

src/Cameras/babylon.arcRotateCamera.ts

@@ -138,7 +138,7 @@
         constructor(name: string, alpha: number, beta: number, radius: number, target: Vector3, scene: Scene) {
             super(name, Vector3.Zero(), scene);
 
-            if (!this.target) {
+            if (!target) {
                 this.target = Vector3.Zero();
             } else {
                 this.target = target;