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