Actually an amino acid supplement drink or protein shake (low carb) is the best pre-workout source of fuel to burn fat/keep muscle in mornings on a empty stomach. The amino/protein complex will help retain the muscle, while you're burning pure glycogen (stored carb/sugar) or fat as you jog or w/e
If you eat any type of carbs before morning cardio you'll be burning the carbs from the food before you even tap into your bodies stored glycogen. Thus taking it a much longer cardio routine before you even start burning fat. You body does not start burning fat as energy until your stored glycogen is depleted. Hence why keto diets are most effective and rapid fat loss
The key is to keep your carbs low at dinner (veggies), when you go to sleep your glycogen will be lower, when you wake and hit the cardio on a light pure protein stomach your glycogen will still be low and your workout wont take long before you start burning body fat. Post workout throw some oats in a protein shake and replenish some glycogen so you don't feel like shit all day.
I never eat carbs preworkout cardio to burn fat unless im doing HIIT or a long lift/cardio session