Run formatting only if files changed. Echo message if pylint fails.
Hey @artbataev , could you point me to an example that motivated this PR? > Hey @artbataev , could you point me to an example that motivated this PR?

@ko3n1g, I added the following changes:
- run actions only if there are changed files. This is an extremely rare case, but we should handle this (if someone deleted `*.py` file - the action will fail). I can demonstrate this easily. Demo: https://github.com/NVIDIA/NeMo/actions/runs/11709698596/job/32614299397?pr=11190 in https://github.com/NVIDIA/NeMo/pull/11190
- I added an explicit message to the developer about what to do if `pylint` fails. I already got a personal message about it in Slack. 
- `git diff` can fail if the PR target is different from the main, e.g., the release branch. But this will only be the case after some release branch is cut from the current main. Using `${{ steps.changed-files.outputs.all_changed_files }}` is safe and maintainable (and contains only changed *.py files).

# What does this PR do ?

- fixes the issue when code actions are executed when files are deleted
- explicitly explain to developer how to fix pylint errors

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