Packets of sweetner fall into the category of foods that are less than 5 calories per "serving" so manufacturers are allowed to round down to 0 calories. They do have minimal calories and it can add up (a cup of Splenda is about 90 calories if, for example, you bake with it).
But unless you're going nuts with the packets, relax and don't sweat it! All food labels are rounded off and calories given in Fitday are estimates. None of us are going to be able to precisely figure out exactly how many calories were consuming in a day. So long as it's a reasonable estimate, you're fine.
And for what it's worth, I've been at this for almost seven years and have never counted the calories in sweetner.