Previously we returned an error when the interpolation failed in
`process_completion_response`.
However, it is not an error when interpolation returns `None`. That just
means that the predicted edits can be discarded, because the user typed
something that is not a subset of what the model predicted OR if the
model responds with a no-op.
```
2025-01-29T09:44:30.221135+01:00 [ERROR] zeta prediction failed

Caused by:
    Interpolated edits are empty
```

Release Notes:

- N/A