Fixed possible issue during importing usd file in USD nodetree.

[bnagirniak]

PURPOSE

Possible issue during importing usd file in USD nodetree

  File "D:\amd-gpuopen\BlenderUSDHydraAddon\src\hdusd\engine\viewport_engine.py", line 521, in _sync
    self.render_params.clearColor = world_data.clear_color
Boost.Python.ArgumentError: Python argument types in
    None.None(RenderParams, tuple)

EFFECT OF CHANGE

Fixed possible issue during importing usd file in USD nodetree.

TECHNICAL STEPS

Added check is not None in WorldData.init_from_stage()