If a value is NULL, how to show a 0 insead?

I have a sample dashboard, where I'm trying to show what % of a neighborhood came in either 0 times a week, 1 times a week , 2 or 3 times a week, or 4 and 5 times a week. There is a parameter, p.Choose Metric that allows the user to choose. However, if a value is NULL or blank it means that no one from that neighborhood came in. So the metric should be 0 for 1x weekly, 2x/3x weekly, and 4x/5x weekly. For 0x weekly it should be a 100%. Right now, it just shows a blank space. Any help would be greatly appreciated. Thanks!