property check fails in style expression

I am going to show points on Point Cloud according to Classification.
If "Classification" property exists, style expression works fine below
['${Classification} === ' + id, color]

But if "Classification" property does not exist, I get this error message.
RuntimeError: Style references a property "Classification" that does not exist or is not styleable.

So I tried to like this in expression.
"defines" : {
"Classification" : "${Classification} !== Undefined ? ${Classification} : -1"
},

But still get error with this message
DeveloperError: The style is not loaded. Use Cesium3DTileStyle.readyPromise or wait for Cesium3DTileStyle.ready to be true.

I saw Undefined is avilable to use and @lilleyse mentioned at #4637 that it is available to use {MyProperty} !== undefined.
Also I checked it is available to use Undefined at https://github.com/AnalyticalGraphicsInc/3d-tiles/blob/master/specification/Styling/README.md#reference-point-cloud-style.

If Cesium doesn't support this, I think there should be built-in function to check property in PointCloud.

Start with the point-cloud style example in the issue and compare its undefined-property behavior with the linked point-cloud styling specification. Then inspect Cesium3DTileStyle.readyPromise and ready handling around the reported DeveloperError. Done would require a decided, tested behavior for missing properties or a clear resolution that the requested check is unsupported.

Written by the indexing model from the issue text.