Exporting unused parts

[negativeview]

To reproduce: Create a model that has three materials. Do not assign one of the materials to the model. Export said model.

What happens: In g3dj, there is an entry in "nodes" and "parts" for the material. Parts has no vertexes. If you try to import said model, g3dj will give you an error saying there's an invalid part. g3db gives a more cryptic error, but I assume it's the same thing under the hood. Manually deleting the node and part in g3dj makes the model work.