There is no clear consensus. The few studies done suggest the opposite of what you are saying - that the glycogen depleted state your body is in when you wake up leads to burning fat before muscle. That said, most physiologists suggest you should eat something before working out. THAT said, most bodybuilders suggest doing "fasted AM cardio." I tend to go with the bodybuilders' advice as they are the ones actually putting their ideas into practice and figuring out what works and what doesn't.