[Feature Request] Add a check if Token is empty or not and show better error message

[alexander-schranz]

Subject of the issue

I did run into #871

fatal: could not read Username for 'https://github.com/': No such device or address
  Error: The process '/usr/bin/git' failed with exit code 128

And sadly github doesn't allow some debugging if all is given correctly to the action. It would be nice if the action could check if token is not empty and show a more friendly

Steps to reproduce

Try to use the action without a token. The error is a little bit confusing.

[peter-evans]

Thank you for raising this. I've added a check for the token input and released as v4.2.3 / v4.

[alexander-schranz]

@peter-evans that is really great thank you!