Well basically you cannot save a form in SharePoint without filling all required fields... 

To fire a workflow one the file is checked in, create a people variable, but don't set it. Then add a filter action in the beginning of your workflow and set a rule: checked out to is equal (your person variable).

That will ensure your workflow to pass the filter only when checked out to is empty what means that the file is checked in

Regards, 

Tomasz 

Hi!

Have you had a chance to test the solution I described? Have it worked for you?

Regards, Tomasz 

Thanks for the suggestion Tomasz. This solution worked.