ff0a2d6dbf
Initialise the Action's `last_slot_handle` field to its default value.
The DNA 'defaults' system doesn't do this automatically when versioning
legacy Actions, so now it's done explicitly in the versioning code.
It's fine to only apply this to the existing versioning code, and not as
a new versioning rule. Existing slotted Actions should just keep that
field to whatever it is now.
The default value was introduced in be921b8ddb
Pull Request: https://projects.blender.org/blender/blender/pulls/131498