Fix: Ensure the resource is not None before we call getUrl

[ogios]

The resource can potentially be None because of this piece of code in tesseract:

https://github.com/tesseract-robotics/tesseract/blob/821da48bf9b9a16a4cfeeffb39631b9797bbd6d8/tesseract_geometry/include/tesseract_geometry/mesh_parser.h#L403

  return createMeshFromAsset<T>(scene, scale, nullptr, normals, vertex_colors, material_and_texture);

It will raise error 'NoneType' object has no attribute 'getUrl' and break the program.