Allow empty targets

[mrmlnc]

Hello, @tclindner,

First, thanks you for this project 🎉

Description of change

I have a process in which I want to check only added packages (packages that did not exist before). I trying to do it with the following commands:

"scripts": {
   "get-added-packages": "git diff origin/master...HEAD --name-only --diff-filter A packages | grep -i \".*package.json$\"",
    "check-new-packages-deps": "npmPkgJsonLint $(npm run --silent get-added-packages)"
}

In this case I cannot guarantee that the output from the get-added-packages command will contain at least one file.

So, this PR adds the ability to not throw an error if no target is specified at startup.

Checklist

• Unit tests have been added
• Specific notes for documentation, if applicable

[tclindner]

Neat 👍 Cool use case, @mrmlnc!

P.S. Sorry for the delay. I was traveling ✈️