Normally, you should not have an empty stomach when taking antibiotics. I did that before because by mistake the pharmacy told me that it is ok to take it even before meal. I ended up having blood in my stool. So, not advisable to fast during medication.
I know of one guy who screwed up his gut biome just by water fasting while taking medication and it took him 4 years to get his old gut Flora back. This is a one time anecdotal thing, but I’d be very careful with this.
Finish your medication, reestablish your gut Flora and then dry fast. Fasting if done right has a very healthy and protective effect on your gut biome. Maybe it would help you while taking medication, maybe not. I’d rather be safe than sorry.
Good luck and stay healthy