Update PathVisualizer.js
Update PathVisualizer.js

Keeping around the polylineCollection in the scene if there are no items to visualize hurts performance.  In our use case it is a ~15% hit.
CesiumGS/cesium,YonatanKra/cesium,YonatanKra/cesium,likangning93/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,progsung/cesium,likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,YonatanKra/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,progsung/cesium,YonatanKra/cesium,AnalyticalGraphicsInc/cesium