A 15cm celery stalk has 19 kilojoules (five calories). Your body uses about 10% of the celery’s total kilojoule value to digest it. This is called the thermic effect of food. In practical terms it’s just one or two kilojoules.
The only thing with “negative calories” is cold water. That’s because it needs to warm up to body temperature before it can be absorbed. That “heating” uses up some kilojoules. Water itself contains no kilojoules, so the overall effect is “negative”.