default text if summary is null or empty

[ievdokdm]

Fix: flutter/flutter#184360

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🤖 Hi @ievdokdm, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[github-actions]

## 📋 Review Summary

This Pull Request addresses an issue where an empty summary (not null) in a presubmit job would be displayed as an empty box instead of default details. The fix correctly uses a null-aware and empty-check pattern to show default information in these cases.

🔍 General Feedback

• Robustness: While the fix correctly handles null and empty strings, consider using trim().isEmpty to also handle cases where the summary consists only of whitespace characters.
• Maintenance: A minor typo ('bellow' instead of 'below') was noted in the related _getDefaultJobDetails function's return strings, which should be corrected for better user experience.
• Testing: Adding a test case for the empty string scenario would ensure the fix is robust and prevents future regressions.