In situations where you've picked up your food items yourself and received no table service or other personalized assistance, it's entirely acceptable not to leave a tip. Tipping is generally a way to show appreciation for good service, and when there's no service involved, there's no obligation to tip.
If the restaurant asks you if you want to leave a tip during the take-out order process, a simple and polite response would be: "No, thank you. I picked up the order myself, so there was no service provided." This response will make it clear that you're declining to leave a tip because there was no service, and it should be understood by the restaurant staff.
Tipping culture can vary depending on the country and region, so it's always a good idea to be aware of local customs and expectations. However, in the scenario you described, it's generally accepted not to tip for take-out orders where no service was provided beyond preparing the food.