"Empty Stomach" has to be defined. True empty stomach (technically called postabsorptive state) is upon waking in the morning. Beyond that, there's a continuum of fed versus fasted. I think it's optimal to be in the mid-absorptive phase of a meal during training. That means training should begin 60-90 minutes after a large meal (if it's legs or ab training, then up to 2 hrs or so if you're prone to upchucking). If it's something more quickly digested like a small meal or shake, then within 30 minutes preworkout is tolerable for most. There's also nothing wrong with sipping a prot/carb shake throughout pre, during, & postworkout, especially if you train longer than 90 minutes. The superior benefits of absorbing prot & carbs during training instead of training fasted are well documented.
^^With all that said, I personally can't reconcile the idea of going into a training bout on an empty stomach just so I can take X Y or Z supplement.