A nuke will be pretty devastating in space. Indeed it will not have massive blastwave but the heatwave and the radioactive rays are still there and utterly devastating. The principles of hear radiation work in space as demostrated by the radiators in the shuttle orbiter's cargo bay doors. Another way of understanding a nuke in space; at the moment of the explosion the nuclear bomb is basically a microscopic sun the temperature of the fireball is even similar to the temperature on the surface of the sun ~6500 Kelvins. While it does not have the enormous heat radiating are as the sun the nuke will still pretty much melt everything away
Now considering that 50% is blast 35% is thermal radiation made up of a wide range of the electromagnetic spectrum, including infrared, visible, and ultraviolet light and some soft x-ray emitted at the time of the explosion and 15% as nuclear radiation including 5% as initial ionizing radiation consisting chiefly of neutrons and gamma rays emitted within the first minute after detonation and 10% as residual nuclear radiation. Now crossing out the things that will occur in space we can conclude that a nuclear weapon has got the 40% of its capacity in space.