Add check for Device in DMX.fade

[kflak]

It would be great to check if a device is not nil before launching a fade, especially when working with "mixed" project, where light is an optional part of the setup. I could probably sit down and do a PR for this next week, when I have a bit more time, but it would be good to get a couple of ideas on how to go about it in the most generic way possible. I'm thinking just to wrap the body of the fade method with an nil check for device. What do you think?