The idea that it's good to exercise on an empty stomach is, first off, wrong.
It's based on the bad science of thinking that if you have no food calories to burn, your body will burn fat for fuel.
This is just another bs shortcut people want to believe.
Losing fat doesn't depend on whether you do exercises on an empty stomach or what exactly your body burned for fuel during your 20 minute workout, fat loss depends on maintaining a caloric deficit. In fact, if you work out on an empty stomach, especially in the morning, if I remember correctly, your body burns a higher percentage of lean tissue for fuel (muscle) and we all know that ain't good.
Your body needs food to function properly, even to burn fat properly.
Now, if people are talking eating a big meal before a workout, I would agree that it's not a good idea. But I always make sure to have eaten well earlier, and to have a good pre workout, usually a shake.
I would advice you a cup of coffee. Maybe even 2.
And just placing your meals with enough time arround your workout. Which is a little different for everyone.