I remember reading once that if food has negligible calories, like less than 5 per serving or something, that they could list it as 0 calories. It's like Pam - if you only use one serving, you'll be good to go. If you eat that entire jar of pickle relish, you're probably adding on a good number of calories. I would say it's safe to add to the tuna. Enjoy your lunch!
I don't have Hines, but I do have Vlasic, which is 5c/0f/1c, and Steinfeld's, which is 0c/0f/0c. Now Steinfeld's sweet relish is 15c/0f/3c. So stick with the dill, and don't eat the whole jar!