Zoom error with globe disabled and viewing model from below


1. View tileset from below
2. Try to zoom on.

```js
var viewer = new Cesium.Viewer('cesiumContainer', {
timeline:false,
animation:false,
vrButton:false,
sceneModePicker:false,
infoBox:true,
skyBox: false,
scene3DOnly:true
});
viewer.scene.globe = undefined;

var tileset = viewer.scene.primitives.add(new Cesium.Cesium3DTileset({
url: '../../../../Apps/SampleData/Cesium3DTiles/Classification/Photogrammetry/tileset.json',
maximumScreenSpaceError : 1.0,
maximumMemoryUsage: 4096 // GPU Memory
}));
tileset.readyPromise.then(function(tileset) {
viewer.zoomTo(tileset, new Cesium.HeadingPitchRange(0, -1.0, 0));
});

viewer.homeButton.viewModel.command.beforeExecute.addEventListener(function(commandInfo) {
// Fly to custom position
viewer.camera.flyToBoundingSphere(tileset.boundingSphere);
// Tell the home button not to do anything
commandInfo.cancel = true;
});
```

Run the provided reproduction with the globe disabled and inspect the viewer.zoomTo and camera.flyToBoundingSphere paths while viewing the tileset from below. Trace where zooming produces the error and add a regression check if an existing camera test covers this behavior. Done means zooming from below no longer errors in the supplied setup.

Written by the indexing model from the issue text.