The cause was a missed case from #130440 where
`use_nla_remapping = true` was being unconditionally passed to
`fcurve_to_keylist()` rather than determining the value with
`ANIM_nla_mapping_allowed()` as it should have been.
The fix is simply to use `ANIM_nla_mapping_allowed()` instead of
unconditionally passing `true`.
This PR also changes an outdated comment to properly explain why `adt`
was null in the first place (which is precisely to catch bugs like
this).
Pull Request: https://projects.blender.org/blender/blender/pulls/147957
63d65bebf0