THere is NO ISSUE with putting dead shrimp or fish food in the tank...I put it in to feed my fish, I can put it in without fish. There is nothing wrong with it.
The potential issue is this:
The bacteria die back to populations that can use available ammonia in the system. If, suddenly, you increase that ammonia load you could out pace the ability of the existing bacteria to use it, and that results in a spike in ammonia. I have SEEN this happen. IMO, better safe than sorry - fish food or shrimp, whatever...cheaper and easier to deal with than "cheap" fish you may not want (if damsels) or "relatively cheap fish" that will still ding the wallet if they die. I don't know about you, but I don't like to flush $10 down the toilet for any reason.