fix: guard against None compositor node group (#18)

[SAY-5]

Resolves #18.

get_compositor_node_group() returns None when a scene has no compositing node tree (e.g. when scene.compositing_node_group exists but is unset, or scene.node_tree is None). The compositing scenes panel called .name on the result unconditionally, raising AttributeError: 'NoneType' object has no attribute 'name' whenever the panel was drawn while the active scene lacked a compositing tree.

The reporter (Cristóbal Vila) hit this on Blender 5.1.1 / macOS when pressing Alt-R in the Shader Editor with the Matalogue Tree Panel open, and also when simply selecting different objects. The traceback points to the line in MATALOGUE_PT_compositing_scenes.draw that accesses .name on the result.

Stores the result in comp_group once and guards both .name and the later .nodes access in the same function.