You could drink green tea on an empty stomach but it's not recommended to drink green tea on an empty stomach since it can cause you to have an upset stomach.
It's best to eat something and then drink the green tea on a full stomach so you can avoid getting an upset stomach.
Green Tea contains polyphenols which increases your stomach acid that can cause nausea, a burning sensation in your stomach or even constipation so it's best to drink green tea between or after each meal.
If you don't want to risk an upset stomach you must drink green tea between or after meals.