The ground is not necessary, it is merely a safety measure in the event current passes through the light fixture and you are in contact with it (not the current for the light itself, an outside source of current such as lightning or being energized by another loose live wire). The current would follow the path to ground (via the copper ground wire) instead of running through your body to the ground. It is EXTREMELY unlikely you will ever encounter a problem with not having a ground on a light source. Just use the cord you have and don't lose any sleep over it.