fix: potential panic in trace utils

[letmehateu]

Describe your changes

• TraceFragment::len() method: Added check for empty data before accessing self.data[0]
• The method now safely returns 0 if the fragment contains no columns
• Updated documentation to match the new behavior

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Commits are signed.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.
• Updated `CHANGELOG.md'

[github-actions]

Automated check (CONTRIBUTING.md)

Findings:

• Link an issue in the PR body (e.g., "Fixes Implement SHA256 in Miden Assembly #123").
• Add a short Rationale explaining why the change is needed.
• Change appears to be a trivial code-only edit without tests; may be batched internally.

Recommendations:

• Consider adding a Test plan or clear review steps.

Next steps:

• Link a relevant issue (e.g., "Fixes Implement SHA256 in Miden Assembly #123") and ensure it is assigned to you.
• See CONTRIBUTING.md for expectations.
• If this is a false positive, comment: /quality-review.

[bobbinth]

Thank you - but we do not accept PRs w/o associated issues.