By Jonathan Landay FRONT LINES NORTH OF KHERSON, Ukraine (Reuters) – The Ukrainian mortarmen, hands cupping their ears, turned away from the green tube angled towards their Russian foes entrenched less than a kilometre away as one of their comrades hauled on the firing cord. The crew had expected the flaming blast of a 120 mm fin-tailed cylinder of steel-enclosed high explosive to arc into the grey sky toward a bevy of Russian armoured vehicles. Instead, they got only a dull click. Another dud round. “It doesn’t happen often, but it happens,” Hennadyi, 51, muttered to a visiting Reuters report…