v2.1.x Chore - Consider using more robust error handling when combining coverage files.

[reactive-firewall]

Consider using more robust error handling when combining coverage files. The current approach might miss some edge cases where .coverage.* files don't exist.

Originally posted by @codecov-ai[bot] in #361 (comment)

goal This is just a chore for when next refactoring the tests/check_integration_coverage script.