You have to keep in mind the bane of all introductory chemistry students: excess precision and rounding. Every measurement has error bars, even if those error bars are implicit. It's senseless to report a value that has (much) more precision than your error bars.
A value that's measured as zero isn't necessarily exactly zero. ... it's only "mostly zero".
The particular rounding rules that are used in nutritional labels depend on the specific regulatory agency, but in the US, the FDA provides rounding rules for nutritional labels. It counts anything less than 5 (dietary) Calories per serving as being zero. Likewise, anything with less than 0.5 g of protein, fat or carbohydrate can be listed as having zero grams.
This is how Tic-Tac breath mints can be advertised as being "zero calorie" despite being made almost entirely of sucrose (table sugar): a serving contains less than 5 Calories, and so can be rounded to zero on the label.
In your case, there's a few things which could contain calories. Carmel color is made from sugar, and may contain residual saccarides. Aspartame is a methyl ester of a dipeptide, which is converted in the body to its constituent amino acids, providing a small amount of calories. (Gram for gram, aspartame would contain almost the same amount of calorific energy as sugar does. It's about 200 times sweeter than sugar, though, so on an equivalent sweetening basis it has much fewer calories.) Depending on what the "natural flavor enhancer" is, that might also contain carbohydrates, proteins or fats. (Natural flavors are often fats or fat-soluble, so flavorings often contain trace amounts of dietary fats.)
For a single serving (and for 100 ml) there's only a small amount of carbohydrate, only a scant amount of amino acids, a negligible amount of fat, and an insignificant number of calories. They're a rounding error. But it is true that if you had enough of the drink, you could indeed have an appreciable number of calories, carbohydrates, protein, etc. It's just that it would come along with enough water to make actually consuming that amount physically unfeasible.
