Coca-Cola Zero is formulated with artificial sweeteners rather than sugar, making the claim that it contains no sugar a matter of common knowledge by definition.
The claim that Coca-Cola Zero contains no sugar is a matter of everyday observation and product definition for diet sodas, meaning no citation is needed under Step 0 (Common Knowledge). While the retrieved papers discuss artificial sweeteners, metabolism, and health outcomes of diet beverages, none of them contradict or specifically test the basic formulation fact that Coca-Cola Zero is sugar-free.