Viewer didn't load the model --> Should imports, data and schemas be optional in the schema?

Description:
After select hello-wall.ifcx file in the viewer it's didn't load it into scene, tree view and scene are both empty.

File:
Using a fresh cloned repo with latest example file, tested with `hello-wall.ifcx`

Browser:
Tested with Edge, Chrome and Safari, normal mode and private mode, all behaved the same.

Screenshot:

Console error:
```
render.mjs:513 Uncaught (in promise) TypeError: activeLayer.imports is not iterable
at IfcxLayerStackBuilder.SatisfyDependencies (render.mjs:513:36)
at IfcxLayerStackBuilder.SatisfyDependencies (render.mjs:526:31)
at async IfcxLayerStackBuilder.BuildLayerSet (render.mjs:543:18)
at async IfcxLayerStackBuilder.Build (render.mjs:501:18)
at async compose3 (render.mjs:599:20)
at async composeAndRender (render.mjs:895:10)
at async addModel (render.mjs:953:3)
```

Reproduce the failure by opening hello-wall.ifcx in the viewer, then inspect render.mjs around IfcxLayerStackBuilder.SatisfyDependencies and the reported lines 513-543. Determine how missing imports, data, or schemas are represented and verify that the model loads into both the scene and tree view without the activeLayer.imports error.

Written by the indexing model from the issue text.