docker man page: clarify /etc/docker/daemon.json may not exist or empty

[rubo77]

It is confusing: Dockers documentation says you have to CHANGE a flag in that file, although it does not exist.

I know, I shouldn't send pull requests to this docker/docker-ce repository, but acccording https://github.com/docker/docker-ce/blob/master/CONTRIBUTING.md I should report it to https://github.com/moby/moby, but I don't find the code there