Fix#115983: Crash when saving empty Composite output
Blender crashes when the compositor tries to save en empty composite
output through the render pipeline. This happens when no Composite node
exist and no Render Layers node is used, essentially producing an empty
render result.
We fix this by only saving the composite output if a Composite node or a
Render Layers node exist.
Pull Request: https://projects.blender.org/blender/blender/pulls/117129
c60321f6dc