Duplicating an Action stashes the original one on the NLA. The NLA
evaluation code didn't properly handle the special case of an all-muted
NLA (all stashed actions are muted there) when a layered Action was
assigned directly to the animated ID.
The code is now refactored to remove this special handling from the
`animsys_calculate_nla()` function. Instead it now just returns whether it
did anything. The caller can then decide to evaluate the main Action
instead. This ensures that there is only one "evaluate the main Action"
branch in the `BKE_animsys_evaluate_animdata()` function.
Pull Request: https://projects.blender.org/blender/blender/pulls/127569
65728f16e7