fix: generate-testing-matrix step when there are no files to pass to the script

[Kayyow]

Summary

• The latest "Generate testing matrix" workflow step execution is failing on main: https://github.com/nodejs/docker-node/actions/runs/20997554877/job/60358280253
• Fixes this workflow step to handle cases where there are no modified, added, or renamed files
• Adds fallback values (|| '[]') to prevent undefined values from being passed to the genMatrix.js script

Details

When the diff outputs (added_files, modified_files, renamed_files) are empty, they can cause issues in the GitHub Actions workflow. This PR ensures that empty arrays are passed instead of undefined values.

Test plan

• Verify that the workflow runs successfully when there are no file changes
• Verify that the workflow continues to work correctly with file changes

[Kayyow]

Not sure if this PR is relevant actually