Fix a minor bug in CI slack report
# What does this PR do?

#22798 added code to show the difference between 2 CI runs. However, the previous CI run(s) may not yet produced the artifact `test_failure_tables`, and we got `KeyError: 'model_failures_report.txt'` in the last run.

This PR adds some check.
_The documentation is not available anymore as the PR was closed or merged._