Many if not most fitness buffs workout first thing in the morning. I feel it's the best way to start the day right, and to get it out of the way before work and everything else.
A cup of coffee with some sugar for instant energy is all that is needed for weights and calisthenics assuming your daily nutrition is correct for your weight and fitness goals.
Whether that's good advice for a session of HIIT is debatable: Should You Be Working Out on an Empty Stomach? What you need to know about cardio in the age of intermittent fasting.
For morning exercisers, a banana before HIIT will not be much help because it takes a while for fruit sugars and carbs to be digested and available. Hours. The banana will just sit in your stomach. Sugar, on the other hand, is quickly available.