When set height of camera's destination with 0.0,throw a exception in 2d.

[link.](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=ZVDbasMwDP0Vk5cmEJx2ZTC2tIylfdsNUjYYeVFtpTNz7CI7Kd3Yv89tmrGLHoR1fI50pA6IdQp3SGzGDO5YgU61DX86YvFIHMvCGg/KII1S9lEZFiLLnECDd1bi5UlTDgAvi+X98myR9lSPREH9SLZTEulymCEIweOzJS1XPSVOKvOZXFWmBCMFOK+Rg5QLrKHVfmWtXgPdtN5bE49erG2Cn7o1wqsAJIO1PvdbcQENEvBa71c2PhEOIdF5ZeCgZN+OCiAfXmCmvCbbLHBDiC6ejC/4eTqdhsTGfJz0XY5GQ4rSKHd+r3He49eq2VryrCUdc555bLY67OmydSve0HPh3EGZZ4Mol6pjSs6q6M+1q4gJDc6Fn7rVulTvWEXzPAv8XzJtQSqzeeiQNOwPlNfJ/LYHOed5Fsr/Kt+f80fHynwB)
run the following code,then select 2d mode,and click button [Zoom]:
At the same time,the height is close to 0 ,the page crashed without exception message(2d).

## code:
```JavaScript
var viewer = new Cesium.Viewer('cesiumContainer', {
//sceneMode:Cesium.SceneMode.SCENE2D,
terrainProvider: Cesium.createWorldTerrain()
});
Sandcastle.addDefaultToolbarButton('Zoom', function() {

viewer.camera.flyTo({
destination : Cesium.Cartesian3.fromDegrees(108.5,33.5, 0.0)
});
});
```

Reproduce the linked Sandcastle example in 2D mode by clicking the Zoom button, focusing on viewer.camera.flyTo with a Cartesian3 destination height of 0.0. Read the camera.flyTo entry point and related 2D camera handling; done means the operation no longer crashes the page and reports the failure appropriately.

Written by the indexing model from the issue text.