Blender Git Loki

Fix broken versionning after recent refactor of insertion in liboverrides.

rB33c5e7bcd5e5b79 doversion code was incorrectly dealing with 'insert in
first position' case from older blendfiles.

Specifically, a NULL anchor is valid (it means that the new item is the
first of the stored override data, and should be inserted at start of
the list).

Reported as part of T93321.