I think the evidence on this is still inconclusive however diet pop and sugar free drinks still have ingredients that are not good for our health. I think this would come down to a personal preference, but honestly putting those chemicals into your system to on an empty stomach will not have good effects. I have had diet pop and energy drinks during fasts in the past but it ultimately made me more hungry and I felt worse after. That might not be the case for you though. Try a bit if you think it might help prolong your fast but listen to your body.