Fail for empty `install` parameter

[Serge3leo]

For example:

      - uses: msys2/setup-msys2@v2
        with:
          install:

Fail with:

Installing additional packages through pacman...
  C:\Windows\system32\cmd.exe /D /S /C D:\a\_temp\setup-msys2\msys2.cmd -c "'pacman' '--noconfirm' '-S' '--needed' '--overwrite' '*' ''"
  error: target not found: 
  Error: The process 'C:\Windows\system32\cmd.exe' failed with exit code 1

As a workaround, you can add pactoys if you have nothing to do 😉:

    - uses: msys2/setup-msys2@v2
      with:
        # Workaround, `setup-msys2` don't accepted empty string
        install: ${{ case('' == inputs.add-install, 'pactoys', inputs.add-install) }}

https://github.com/Serge3leo/test-empty-install-setup-msys2

[lazka]

Thanks