Model seems to be valid but renders empty with error logged

### Description

A specific model renders empty in model-viewer but displays correctly in other viewers (e.g. three.js editor, Don's old and new viewers, ...).

#### Live Demo
Drop this model into [modelviewer.dev/editor](https://modelviewer.dev/editor):
[plant_3.o.zip](https://github.com/google/model-viewer/files/11158010/plant_3.o.zip)

There's an error logged:
```
space-opera.js:80510 Mesh is missing primitive index association
```

### Browser Affected
- [X] All as far as I can tell