When assigning an Action, and it has only one slot that has never been
assigned to anything before, auto-assign that slot.
This is the last option for the slot auto-selection. It is in place mostly
for backward compatibility in the following situation:
- Python script creates a new Action, and adds F-Curves via the legacy API
`action.fcurves.new(...)`. This automatically creates a slot with
`id_type = 0`, indicating it is not yet bound to any ID type.
- The script assigns the Action to a data-block.
In this case the implicitly created slot is automatically assigned, and
thus the data-block is animated by the F-Curves created through the legacy
API.
Pull Request: https://projects.blender.org/blender/blender/pulls/128892
ee3aea4caf