Fix changed-files for cases where no python files changed

[DavidJFelix]

Problem

When a PR contains no changed python files, but changed-files is set to true, ruff will run on all files, including ones not changed.

Solution

Pass the input through to the script and short circuit to success in cases where both the flag is set and no python files were changed.

[DavidJFelix]

Linking old PR from pre-fork repo: ChartBoost/ruff-action#29

[charliermarsh]

Thanks!