Default lighting not showing up in Solaris and Usdview

**Describe the feature you'd like**
When using the render delegate in Solaris, the default camera light does not affect the scene. This is causing confusion, as assigning a standard surface to any object will result in black renders without a light, yet in other render delegates, like karma, there is a default light.

Start by reproducing the missing default-light behavior in Solaris and Usdview with the Arnold render delegate, using the comparison with Karma described in the issue. Trace the render-delegate entry point involved in camera-light setup and verify that a standard-surface scene renders without an explicitly assigned light.

Written by the indexing model from the issue text.