Shaders linked via primvars removed on AiSceneWrite

**Describe the bug**
When a .usd file is loaded and resaved either via kick or AiSceneWrite directly if a shader is only linked via a custom primvar the the shader is removed from the `ArnoldUnassignedShaders` prim

**To Reproduce**
Steps to reproduce the behavior:
1. download the example [test_custom_shader.usda.zip](https://github.com/user-attachments/files/24525397/test_custom_shader.usda.zip)
2. run the kick comand: `kick -i test_custom_shader.usda -resave test_custom_shader_resaved.usda`
3. the resulting usda file removed the `ArnoldUnassignedShaders` nodegraph

**Expected behavior**
All shaders in and `ArnoldUnassignedShaders` Prim should remain on resaving the USD file via AiSceneWrite

**Used Software Versions**
- Arnold: 7.4.4

**Additional context**
If I remove the line
```
string inputs:name = "myRefShader"
```
The nodegaph remains when re-saving

Start by reproducing the issue with `kick -i test_custom_shader.usda -resave test_custom_shader_resaved.usda`, then trace the `AiSceneWrite` resave path for shaders linked through the `myRefShader` custom primvar. Done means the resaved USDA retains the `ArnoldUnassignedShaders` nodegraph and its shaders.

Written by the indexing model from the issue text.