Translating skydome_light to DomeLight causes error messages in HdStorm

**Describe the bug**
When converting an ass file to USD using the writer, DomeLights in the scene lead to error messages, when the scene is opened in usdview and rendered in HdStorm.

```
ERROR: Usdview encountered an error while rendering.
Error in 'pxrInternal_v0_20__pxrReserved__::UsdImagingGL_GetTextureResource' at line 405 in file pxr/usdImaging/usdImagingGL/textureUtils.cpp : 'Failed verification: ' attr.Get(&asset, time) ''
```

Manually editing the USD file and setting up the texture parameter on the DomeLight gets rid of the error message.

[Example ass and USD file](https://github.com/Autodesk/arnold-usd/files/4190970/skydome.zip)

**To Reproduce**
Steps to reproduce the behavior:
1. Convert an ass file to USD
2. Open the file with usdview

**Expected behavior**
There should be no error messages.

**Used Software Versions**
- Arnold: 6.0.1.0
- USD: 0.20.02
- Compiler: GCC 4.8.5
- OS: CentOS 7

**Additional context**
This is most likely a limitation in UsdImaging/HdStorm, DomeLights will print an error message without a texture file.

Start with the linked skydome.zip example and reproduce conversion from the ASS file, then open the USD in usdview with HdStorm. Compare the DomeLight texture setup in the generated USD with the manually edited USD and inspect the reported UsdImagingGL textureUtils.cpp verification. Done means the converted scene renders in HdStorm without the error.

Written by the indexing model from the issue text.