If you do not ever get dessert I thought I read somewhere you can ask for a bottle of water instead.
Found this information on easywdw.com:
Dessert Substitutions at Quick Service Locations
We have a saying in the online world called “YMMV,” which translates to “your mileage may vary.” Officially, a quick service meal includes the entrée, dessert, and drink. However, you may be able to make substitutions for the dessert portion of the meal, depending on the quick service location and your cashier. Most quick services offer just a couple of desserts, usually in the form of a pre-packaged slice of cake or cookie. You may be able to substitute a bottle of water, kids’ apple slices, frozen lemonade, yogurt, or similar item for the traditional pre-packaged dessert option. Because the Dining Plan is so prevalent now, Disney has expanded the dessert options on many menus to include yogurt and frozen lemonade-type drinks at many location, so substitutions aren’t as much of a hassle as they once were.
Still, you’re not going to see bottles of water on many (any) dessert menus. If you would like to try to substitute an item, simply ask the cashier if it’s possible. You will have the most luck if the item you are substituting either costs the same or less than the item you’re “supposed” to order. Smile, say something like, “None of these desserts sound like they’ll work for us…do you think we could substitute…instead?” There’s no harm in trying and you’ll very likely have mixed results throughout your trip, even if you return to the same counter service, get a different cashier, and ultimately get a different answer.