Upgrade fails immediately - No default datasource found

[vinci72]

Source version: 4.4.17 Home
Target version: 5.0.2 Home

Upgrade fails immediately after ./cells update --version=5.0.2 and subsequent start:

No default datasource found

Confirmed root cause candidate:
Our pydio.json contains:

"pydio.docstore-binaries": {
"bucket": "binaries",
"datasource": "default"
}

However, no datasource named "default" exists anywhere in our config:

• It is absent from the "sources" list logged at startup by
  pydio.grpc.data.sync (we have 16 active named datasources — none called "default").
• There is no "pydio.grpc.data.objects.default" or
  "pydio.grpc.data.sync.default" service block in pydio.json.

By comparison, the sibling services resolve correctly:
"pydio.thumbs_store": { "datasource": "thumbnails" } -> exists, works
"pydio.versions-store": { "datasource": "versions" } -> exists, works

So pydio.docstore-binaries is the only service pointing at a datasource that has never existed in our active config. Under 4.4.17 this is apparently tolerated silently; the 5.x migration process fails hard on it instead of falling back or warning.

This occurred identically both on manual ./cells start and after a full server restart (i.e. via our systemd service, user pydio, WorkingDirectory=/srv/pydio). We reverted via VM snapshot both times rather than confirming the "reset initial configuration?" prompt, to avoid risking the 16 production datasources.

Question: is pydio.docstore-binaries expected to require an explicitly-defined "default" datasource in 5.x, and if so, what is
the supported way to migrate a legacy config that never had one (rather than resetting the whole install)?