Joint names [index] is NoneType

[alexcubor]

Hi! Thanks for the great job! This library has a lot of potential! 🤩
I get an error when I try to convert a model with skeletal animation:

  File "/Users/alexcubor/miniconda3/bin/usd2gltf", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/alexcubor/miniconda3/lib/python3.11/site-packages/usd2gltf/cli.py", line 73, in main
    args.func(args)
  File "/Users/alexcubor/miniconda3/lib/python3.11/site-packages/usd2gltf/cli.py", line 17, in run
    factory.process(stage, args.output)
  File "/Users/alexcubor/miniconda3/lib/python3.11/site-packages/usd2gltf/converter.py", line 356, in process
    skel_id, gltfSkelRoot, joint_node_ids = usd_skel.add_skeleton_rig(
                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexcubor/miniconda3/lib/python3.11/site-packages/usd2gltf/converters/usd_skel.py", line 627, in add_skeleton_rig
    joint_path = Sdf.Path(jointNames[idx])
                          ~~~~~~~~~~^^^^^
TypeError: 'NoneType' object is not subscriptable

Tested on the file dzinka_ac.usdz in this archive:
dzinka_aс.usdz.zip