Treat a non-existent mesos modules directory the same as an empty mesos modules directory

When there are no modules, there is often no module directory. A non-existent modules directory indicates exactly the same thing as not having any modules inside the modules directory.

In DC/OS we have to carry some extra stuff to make sure we always have a existing modules directory even in cases where we don't have any real mesos modules in it (https://github.com/dcos/dcos/pull/849)