Unassigned arnold shaders written to USD should go under the mtl scope

Currently when shaders need to be authored to USD without a material binding, they are saved at the root of the stage. They should at least go under the "mtl" scope

No source file or test is named. Start by locating the Arnold shader authoring path that handles shaders without a material binding and inspect how USD stage scopes are selected. Done means unassigned shaders are written beneath the "mtl" scope instead of at the stage root, with coverage for that case if the existing tests provide a suitable place.

Written by the indexing model from the issue text.