fix loading models without materials

[ikmersia]

Buenas Jesus! Subo un fix de un problema que me he encontrado:

When using onAdd, if options.mtl is not specified MTLLoader.js fails and the model is not loaded.
I encountered this error using .glb file.
when url is not passed, loader.load call is made with url = current navigation path.
When testing in local this value was localhost:3000, which worked well (returned 304 and then object is loaded)
When uploading to production, url value was https://dev.immersia.eu/emplazamiento/45, (note that this is the current frontend path where map is rendered). I have frontend in aws cloudfront, the response is 404 and the model is not loaded

How to reproduce: pass https://dev.immersia.eu/emplazamiento/45, for example, as options.mtl for tb.loadObj function.

My fix:
If material is specified, try to load the material, if not, just render the object without the material.
I tried it and it works.

Temporal fix, just use a url that doesn't return an error value for mtl field (any page with allow any cors origin?), for example:

const options = {
obj: modelUrl,
type: modelType,
units: "meters",
scale: layerScale,
anchor: "bottom",
rotation: { x: 90, y: 0, z: 0 },
mtl: "https://jsonplaceholder.typicode.com/",
};