Prevent access on null CesiumPointCloudShading

[j9liu]

Description

This addresses a bug with point cloud tilesets that are created at runtime. Since CesiumPointCloudShading is a class, not a struct, it's possible for a Cesium3DTileset to be missing one, which is exactly what happens when it is created at runtime. This PR adds checks for a null reference, and also explicitly initializes CesiumPointCloudShading inline for good measure.

Ideally, CesiumPointCloudShading would just be a struct to avoid this, but C# 9.0 does not like structs with non-zero default values. :)

Issue number or link

Fixes #602.

Author checklist

• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
• [] I have updated the documentation as necessary.

Testing plan

1. Start from 02_CesiumMelbourne in the Cesium samples.
2. Create a new script called PointCloudLoader with the contents below.
3. Add the component to any arbitrary GameObject. I just tacked it onto the Getting Started object.
4. Press Play. Ensure that the console is not spammed with errors as the points load in.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using CesiumForUnity;

public class PointCloudLoader : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        var go = new GameObject("PointCloud");
        go.transform.SetParent(FindObjectOfType<CesiumGeoreference>().transform);
        Cesium3DTileset tileset = go.AddComponent<Cesium3DTileset>();
        tileset.ionAssetID = 43978;
    }
}