The code for converting pre-Animato actions was not getting run properly
because `chanbase` (where animation data used to be stored) was
erroneously getting cleared before the relevant versioning code was run.
The root cause was that the code checking whether an action was already
valid as a layered action or not was NOT confirming that `chanbase` was
empty as part of that check (as it is a DNA-deprecated field), which in
turn triggered code that defensively clears `chanbase` (among other
things) when an action is identified as layered.
Note that the conversion of IPO curves and other pre-Animato data
happens quite late in the versioning, even _after_ the "versioning after
linking" stage. This is not introduced in this commit, this is just to
illuminate pre-existing design that might not be entirely obvious.
Pull Request: https://projects.blender.org/blender/blender/pulls/131975
2f84d20455