[ML] Trained Models: fix `NaN` in a progress bar during the download task initialization

[darnautov]

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 were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: 9b759a565b692c5ddf0fe2665f22b0060e8b6c1e

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
ml	4.7MB	4.7MB	+6.0B

cc @darnautov

[kibanamachine]

Starting backport for target branches: 8.16, 8.17, 8.x

https://github.com/elastic/kibana/actions/runs/11970833950

[kibanamachine]

💚 All backports created successfully

Status	Branch	Result
✅	8.16
✅	8.17
✅	8.x

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