Fix #143722: Set a default exposure for Hydra dome/world light

The Storm hydra delegate requests this property and USD will trace, per
the bug report, if it is not present.

This started happening with the library update for 4.4 which is odd.
The code[1] in Storm has existed forever and so has the USD trace for
requesting data from an empty VtValue.

[1] https://github.com/PixarAnimationStudios/OpenUSD/blame/release/pxr/imaging/hdSt/light.cpp#L305

Pull Request: https://projects.blender.org/blender/blender/pulls/143746

source/blender/io/usd/hydra/world.cc

@@ -126,6 +126,7 @@ void WorldData::init()
 
   data_[pxr::UsdLuxTokens->orientToStageUpAxis] = true;
   data_[pxr::HdLightTokens->intensity] = intensity;
+  data_[pxr::HdLightTokens->exposure] = 0.0f;
   data_[pxr::HdLightTokens->color] = pxr::GfVec3f(1.0f, 1.0f, 1.0f);
   data_[pxr::HdLightTokens->textureFile] = texture_file;