I hope that this buyer gets banned as soon as possible. What a scumbag.
I wouldn’t make an empty delivery. It is still against ToS, and it leaves an opening for Fiverr to punish you, as unfair as that might be.
If you get a negative review, I would suggest asking CS to remove it.
From the ToS:
“Feedback reviews will not be removed unless there are clear violations of our Terms of Service and/or our Community Standards.”
I would think spreading a virus could warrant a review getting deleted, as it is criminal behavior.