Avoid generating crdt for UI-only smart wearables

DrDripp was reporting that he had a hard time creating a smart wearable
The code of his smart wearable only generated UI (as happens for most cases), no 3D content. But for some reason, when running `npm run pack` it gererated a main.crdt file
He eventually figured out that he had to manually delete this file from the zip every time
He had created this scene without using the Creator Hub, so the file wasn't added by opening the scene in the editor

I can reproduce this issue like this:

- run `npx @dcl/sdk-commands init --project smart-wearable`
This creates the scene scaffolding. No main.crdt or main.composite anywhere in the project

- Run `npm pack`
Now yes I see a main.crdt in the project, that doesn't include any content. It must be removed manually from the generated zip before submitting the collection.
This file should be conditionally generated only when there's 3D content needed to be rendered. Some Smart wearables might need this, most don't.