One study reflects that 0 calorie drinks may actually cause weight gain:
“On average, for each diet soft drink our participants drank per day,
they were 65 percent more likely to become overweight during the next
seven to eight years, and 41 percent more likely to become obese,”
said Sharon Fowler, M.P.H., faculty associate in the division of
clinical epidemiology in the Health Science Center’s department of
medicine. She presented the finding June 12 in San Diego at the
American Diabetes Association’s 65th Annual Scientific Sessions.
As for assuming they are 0 calories, every country has their own food labeling laws, but in the US anything lower than 4 is considered 0. So yes, technically there are around 0 calories, but as the study shows, there is more to it than that, and you might want to consider avoiding them.
More info on labeling laws in the US by the FDA: