Animated transforms not properly backed to its own xform prim path when exporting to USD

**Describe the bug**
If we have animation on parent xforms, but no visible meshes under the parent transfrom node, those transfrom node won't be write to usd xform prim, or instead all animation xfrom data are baked to mesh.




We would like to reference or add more assets into that parent xform to share the animation based on the expored usd stage, but how to do that if animation is not recored in the correct prim path?

Start by reproducing the USD export with animated parent xforms that contain no visible meshes. Verify that each animated parent is written to its own xform prim path rather than baking the animation onto a mesh; the issue provides no source file or test entry point to begin from.

Written by the indexing model from the issue text.