Guppy's are somewhat know for being nippy fish, so it is not overly surprising. The tail looks fine right now, just keep an eye on it. If it startes to get red on the ends where it was nipped, then you may need to treat it but otherwise I would not worry too much. It's pretty common to see a litle nipping here and there, as long as it is just a little.
However, as I mentioned before, getting some ornaments in the tank will help to stop that from happening. Fish in community tanks need safe places to retreat, something to break their line of vision from each other to allow them freedom from stress if someone gets nippy or in their face - fish can be territorial. With nothing in the tank but themselves there is nothing to break the territory as it were, and nothing to do for entertainement but nip at each other. Please try to get at least something in there, it will help.