Viewer component baseLayer behavior is inconsistent with Cesium

[mfarmer-ara]

On Cesium's Viewer doc, it expects baseLayer to be:

ImageryLayer | false or undefined.

In Resium's Viewer.ts, it is forcing baseLayer to be undefined when it is set to false.

This is causing the map to reach out https://api.cesium.com/v1/assets/2/endpoint to when you want to turn off baseLayerPicker and baseLayer in favor of the ImageryLayer component as Cesium falls back to the Ion behavior if baseLayer is not explicitly false.