When a Blender material has no nodes, its viewport color,
roughness and metallic values are saved as inputs to a simple
USD Preview Surface. This pull request fixes a bug where
the Blender material's viewport color is also saved as
the USD Preview Surface emissiveColor attribute.
This bug was accidentally introduced in #107947.
To reproduce the issue, open the default Blender scene,
turn off nodes on the material and set the material's viewport
color. Export to USDA and notice that the color is incorrectly
set as the emissiveColor input in the USD shader.
Pull Request: https://projects.blender.org/blender/blender/pulls/109138
a6f3afe995