Fix #119316: instancing lamps does not work

The issue was that calling `ensure_geometry_instances` converts all instances to a
geometry, even the ones that can't be converted. The comment already says that
non-geometry instances should stay intact, but that didn't work correctly yet:
```
  /**
   * If references have a collection or object type, convert them into geometry instances
   * recursively. After that, the geometry sets can be edited. There may still be instances of
   * other types of they can't be converted to geometry sets.
   */
```

Pull Request: https://projects.blender.org/blender/blender/pulls/119324