Why coke zero has no sugar but still sweet?

Because they are using artificial sweetener. Opposed to natural sugar like white sugar, brown sugar, or honey, synthetic sugars only taste sweet in your mouth and do not increase blood sugar level.

They display 0g of sucrose in the nutritition facts behind your coke bottle or can.

Artificial sweetener including Aspartame, Sucralose, and Stevia does not increase blood sugar level and really safe for people who have diabetes. Coke Zero contains mix of artificial sweetener Aspartame and Acesulfate Potassium (Ace-K).

Why diabetics have problems with blood sugar levels? Because their body does not produce enough insulin or does not use insulin effectively to regulate the blood sugar level.

What is insulin? Insulin is a hormone that is regulating blood sugar and allows it to be absorbed into energy. If you have insulin resistance, you’ll have more sugars stored in your fat.