bugfix(htree): Handle null parent pivots and correct animation data indexing

[seer-by-sentry]

Fixes CLIENT-3DA. The issue was that: HTree's animation update iterates beyond allocated NodeMotion array, corrupting memory, leading to Matrix3D::Multiply access violation.

• Removed assert(pivot->Parent != nullptr) to prevent crashes when a pivot's parent is null.
• Added a check to skip processing pivots with null parents, preventing dereferencing issues.
• Adjusted nodeMotion increment logic to only advance when a pivot has animation data, ensuring correct indexing.
• Conditionally increments nodeMotion for null-parented pivots if they are within the animated pivot range.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 12700695

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