I don't have a problem with coffee on an empty stomach. I've done it a lot on the Appalachian Trail. However, before my thru-hike I did need something in my belly before drinking (and not just coffee), but that was before I got into intermittent fasting. Intermittent fasting has taught me not to give into that empty-stomach feeling. That feeling does go away, it's almost as if it's more psychological, than an actual signal that your body is telling you, you need food.