NVIDIA: Defensive check for UsdSkel_SkelAnimationQueryImp to avoid crash

[roggiezhang-nv]

Description of Change(s)

Removing animation prim with visiblity update to UsdSkelRoot may cause crash to hydra code.

Link to proposal (if applicable)

Fixes Issue(s)

Checklist

• [x] I have created this PR based on the dev branch

• [x] I have followed the coding conventions

• [x] I have added unit tests that exercise this functionality (Reference: testing guidelines)

• [x] I have verified that all unit tests pass with the proposed changes

• [x] I have submitted a signed Contributor License Agreement (Reference: Contributor License Agreement instructions)

[roggiezhang-nv]

@nvmkuruc for vis

[jesschimein]

Filed as internal issue #USD-10701

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