Handle `noData` values when accessing metadata values

[jjhembd]

The schema for 3DTiles metadata specifies a noData value to signal entities where a metadata property is not present.

#10520 exposed the noData and default value via the metadataClass struct inside a CustomShader, so that CustomShader authors can write their own code to handle these values.

We want to handle this automatically when accessing metadata properties. If the noData value is encountered, accessing the property should automatically return the default value from metadataClass.