When people train in the AM, whether lifting or cardio, I am often asked if they don't eat, is it bad?
So the answer is a combo of intensity and energy. When we wake up, we are in a fasted state from sleeping all night. This means our blood sugar is usually low. Depending on what you ate the night prior, your glycogen levels could be low too.
The biggest issue of training on an empty stomach or fasted, is that you could go hypoglycemic and either not perform as well as you would normally, or worse, black out. The intensity level can influence this as well. The higher the intensity, the lower the blood sugar.
So to avoid, get something in you before and/or during training. My go-to is 1/2 scoop whey protein and saltine crackers. Find what sits well and sustains you. If you have more time to digest, then you can get away with eating more and maybe adding a little fat too.