Write empty candidates file if no sources accepted

This is part of the solution for #25.  However, there will still be an error when trying to load an empty candidate file, so the issue should not be closed.  The fix for the error is pretty simple, but to avoid conflicts I want to do it after some changes I am making in the AstromWorkload class as part of #8 are merged in, so it will be a separate pull request after that work is merged in.