Fixed a server problem when no solution found.
(@fhermeni : NullPointer exception generated by  the algorithm :
Problematic Line : JSONObject responseSolution = planConverter.toJSON(plan);
Configuration : http://localhost:8080/?cfg=%5B%5B%5B%22N0%22%2C8%2C7%2C%5B%5B%22VM2%22%2C6%2C5%5D%5D%5D%2C%5B%22N3%22%2C8%2C7%2C%5B%5B%22VM5%22%2C3%2C3%5D%5D%5D%5D%2C%22offline(N3)%3B%22%5D

)