Blender Git Loki

Fix T70573: Crash on enter Cycles render preview with 'Scene world'
disabled and no world assigned to scene

BlenderSync::sync_world still relied on a blender world (mixes the world
viewport color with the studio light), for now just take black if no
world is present.

Maybe we should we use the theme color in the future here (seems eevee
does this in that case) -- we'd have to pass down `b_userpref` from
`BlenderSession::render` down to `sync_data > sync_shaders > sync_world`
then afaics.

Reviewed By: jbakker, brecht

Maniphest Tasks: T70573

Differential Revision: https://developer.blender.org/D6005