fix: check if log exists before trying to upload it

[reubenmiller]

Proposed changes

The tedge-mapper-c8y feature controlled by c8y.operations.auto_log_upload=on-failure, uploads an operation log based on the given condition. This PR just adds an additional check to only do the upload if the log file exists to avoid scenarios where an event is created with no attachment...this typically occurs when running the tedge-agent on a child device, and the mapper does not yet have access to the related operation log.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Additional details and impacted files

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[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
506	0	2	506	100	1h27m16.270048999s

[didier-wenzek]

Approved