Fix the code that decides whether to create a new F-Curve in a legacy or
layered style. The old code went for layered if:
- The experimental feature is enabled AND the Action is already layered.
The new code goes for layered if:
- The experimental feature is enabled AND the Action is empty, OR
- The Action is already layered.
Pull Request: https://projects.blender.org/blender/blender/pulls/128036