[ML] Trained Models: fix NaN in a progress bar during the download …

…task initialization (elastic#201221)

## Summary

When we request a status of the download task right after initializing a
download, `total_parts` might be returned with a `0` value, hence
dividing by 0 for the progress bar causing in `NaN%` value rendered.
This PR adds a fallback to `-1` for that, to make sure we render 0%.

### Checklist

- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

x-pack/plugins/ml/public/application/model_management/models_list.tsx

@@ -697,7 +697,7 @@ export const ModelsList: FC<Props> = ({
                     <>
                       {downloadState
                         ? (
-                            (downloadState.downloaded_parts / downloadState.total_parts) *
+                            (downloadState.downloaded_parts / (downloadState.total_parts || -1)) *
                             100
                           ).toFixed(0) + '%'
                         : '100%'}