Describe the bug
Not sure if this a bug or by design but the action does nothing and exists with 0 if the python-version input is empty. IMO nonzero exit would be better suited.

To Reproduce

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/setup-python@v2
        with:
          python-version:

Example run: https://github.com/jerry-git/gh-actions-playground/runs/3600040805?check_suite_focus=true