App: non-standard optional bone configuration avatars not working

[Vianvolaeus]

Testing with METATRAVELLER avatars results in the equip issue we have encountered with #1744 and #1521 where it resets to origin and does not equip.

I have advised their team on VRM practices, but even with a VRM I've made using their data (rigged model, vrm valid) the error still occurs, so I think there's something on our end that's not working - I suspect it's the fact that the model's rig doesn't contain Little Finger bones:

Note - these are considered optional bones in VRM spec.

The following error occurs on equip:

notably:

Ahad or myself have the test VRM, since it is a non-released project I won't share the data here directly.