What you're thinking of re caffeine and insulin is that is can increase insulin resistance but like Anthrax and Mr. X said that is a minor side effect.
caffeine does not in itself create an insulin spike (right, glucose is needed to do this), but it does effectively amplify the spike when taken with food. This is why the keto diet excludes caffeine. For coffee on an empty stomach, this isn't a problem, notwithstanding the fact that there might be an endoscopy sometime soon in the future....