Use a default commit message if one isn't provided

[fregante]

Is your feature request related to a problem? Please describe.

Currently commit_message is a required input. Given how verbose workflows can be, I prefer not having to specify any inputs when possible.

Describe the solution you'd like

If commit_message isn't specified, it should use a default message. Anything generic works. The "Apply automatic changes" message suggested by the readme would also be fine.

I'm open to submit a PR for this.

[stefanzweifel]

The idea behind this, is that you can simply add this line to your workflows and be done with it, right?

- uses: stefanzweifel/git-auto-commit-action@v4

I think thats actually not a bad idea. "Apply automatic changes" might be a good starting point.
However I would like to encurage users in the README to set their own value.

Feel free to PR this.
Updating the following lines accordingly should do the trick:
https://github.com/stefanzweifel/git-auto-commit-action/blob/master/action.yml#L7-L9

[stefanzweifel]

Has been implemented in #110.
I will tag a new version soon.