NurbsCurve cannot be exported properly along with UsdSkel

**Describe the bug**

Hi,

When we are doing UsdSkel testing and found that if the NurbsCurve is attached to the joint in Maya, the exported USD file seems broken.

I have attached the test Maya scene [usdskel_nurbs_curve_test.ma](https://github.com/Autodesk/maya-usd/files/11295696/usdskel_nurbs_curve_test.ma.txt) (renamed the extension to ".txt").

The skeleton setup in Maya is like this:

```
'group1' -> the skeleton root
'joint1|joint3' -> nurbsCircleShape1 [nurbsCurve]
'joint1|joint2' -> pSphereShape1 [mesh]
```
And the exported USD file content of the NurbsCurve does not have any skeleton attributes comparing with the sphere mesh:


As a comparison if both of the joints are meshes like this (replaced with a cube mesh):


And we got the expected skeleton attributes in USD file:

That seems like a bug that only **mesh** is supported.

**Steps to reproduce**
Steps to reproduce the behavior:
1. Launch Maya and load the attached scene file
2. Click 'File' -> 'Export All'
3. In the export dialog, enable option "NURBS Curves", set "Skeletons" and "Skin Clusters" to be "All (Automatically Create SkelRoots)"
4. Export and see the USD content

**Expected behavior**

The export NurbsCurver should have the correct skeleton attributes.

**Attachments**
[usdskel_nurbs_curve_test.ma](https://github.com/Autodesk/maya-usd/files/11295696/usdskel_nurbs_curve_test.ma.txt)

**Specs (if applicable):**
- OS & version: CentOS 7.8
- Compiler & version: gcc 6.3
- Maya version: Maya 2022.4 / Maya 2023.2
- Maya USD: Offical v0.23.0 / v0.22.0 / v0.21.0
- Pixar USD: Official v0.21.8 / v0.22.11