Maybe you get lunch with your coworkers, and it's not practical to plan it ahead of time. Or maybe you just want to plan dinners, and leave the rest up to whatever you're feeling in the moment.
To have a blank meal, you'll first want to create a Custom Food with a rough estimation for how many calories/macronutrients you'll eat in that meal (here's how to create a custom food). This will let you keep the same nutrition targets, and help balance the rest of your meals' targets around this placeholder meal. Here's an example:
To get this placeholder to show up as the only food in your lunch meals, you'll want to set it as a recurring food set to recur always. Then check off Only use recurring foods for this meal to prevent the generator from adding other foods.
Then the next time you regenerate a day or week, the Lunch should always just have your placeholder in it, and the rest of the meals will generate around your placeholder.
If you want to skip the placeholder step, you can also just search the food database for something. If you think you'll usually be getting takeout for example, try searching our restaurant foods for something similar to what you usually get, and add that instead. You could also just search for "water" and add it, but keep in mind that this will redistribute the nutrition for that meal into your other meals.
One final suggestion: after you eat the meal with your placeholder, you can swap in what you actually ate to better keep track of your intake. Just delete that food from your plan, and then drag in what you actually ate from the Search Foods sidebar. You can delete a food or clear a meal from the 3-dot menus on foods and meals.