Don't fail when no liftover Model is required

[MattWellie]

Fixes

• Fail: https://batch.hail.populationgenomics.org.au/batches/445029/jobs/2
• I wrote this (and the corresponding unit test) so that any no-op liftovers fail. That's incorrect - we're happy to flag that there were no liftover methods, but the assumption is that they're not required
• These would fail during model validation if there was a need to change the data model, so don't fail pre-emptively

Proposed Changes

• If there's no liftover method, don't fail, just pass the data back unchanged

Checklist

• [ ] Related Issue created
• [x] Tests covering new change
• [ ] Linting checks pass