Handle no scene.world in a friendlier way

[Dragorn421]

Illustrated by #326

Sometimes (when???) the scene.world (which stores scene lighting settings in vanilla blender and additional related props for fast64) is None

This prevents materials from working (they appear pink according to whatever causes that default behavior) and any exporting fails with a cryptic message

The fix is simply to create a world in the scene world properties

Fast64 should handle a missing world better
(I'm not sure on the specifics of how)