The CPU compositor does not recognize viewer groups inside node groups
unless a viewer node exists at the top level node group. This is caused
by bad logic when registering viewer node at the node operation builder.
And the fix is just to correct the logic to always register viewers if
no active viewer already exists, which can then later be overwritten if
a viewer node that takes precedence was discovered.
Pull Request: https://projects.blender.org/blender/blender/pulls/122869
d66e3f899a