I do not think it matters in terms of caloric balance. However, there may be an advantage do doing cardio first thing in the morning before breakfast if another lifting session will follow later on in the day. The reason is that most of the calories burned from pre-breakfast cardio will be mostly from stored fat and also from stored glycogen rather than most of them coming from a previously ingested meal. This will reserve more of the calories you will receive from food to be used for your upcoming lifting session.
For example, suppose I do a pre-breakfast cardio session where I manage to burn 600 total calories with 400 of them coming from stored fat and 200 from glycogen. If I then eat a 500 calorie breakfast immediately following the cardio session, 200 calories will replace the burned glycogen and 300 will remain to be used later in the day. The remaining 300 will likely be stored as extra glycogen rather than fat due to the same principle athletes use to carb
up before a competition.
Now suppose that I eat 500 calories for breakfast and then do the same 600 calorie cardio session with 200 coming from stored fat and 400 coming from my meal and stored glycogen. Now, only 100 calories of breakfast remain for use later in the day.
Therefore, if I do cardio before breakfast, I will have additional glycogen stores with which to perform my afternoon or evening lifting session. This will allow me to possibly generate greater intensity during my weight training session.
The above situation is assuredly more complex than I made it out to be, but I think the concept is valid.
Concerning cardio intensity, from my personal experience, I have little trouble burning 600 calories during a 45 minute pre-breakfast cardio session (I weigh 185lb). It is hard work, but it is still hard work even after breakfast.
Concerning burning up all your hard earned muscle, I don't notice any muscle loss doing cardio before breakfast. This is even with HIT cardio with a max heart rate of 170 bpm. (That does not mean it does not happen in small amounts though). Muscle is the body's least preferred source of energy, and even if you begin burning muscle, the rate that your body can draw energy from stored muscle is quite slow. If a significant amount of energy is coming from muscle, it will be extrememly difficult doing any type of cardio session.