Comments are there to tell you things that aren’t already clear from the code.⠀Things like why it’s done this way (especially if you’d normally expect it to be done differently), or any non-obvious interactions with other parts of the system.⠀And of course function/method comments should be written from the point of view of the caller, explaining what the function does, why and when you’d want to call it, what parameters it takes, what exceptions it might throw and why, what value it returns, and how it relates to any alternatives.
So in this case, it might be good to comment why the method has no implementation, when the interface it’s implementing clearly expects some functionality to go there.⠀For example:
// No implementation, because this class doesn't have any flurgles that need zorping.
(Hmmm, the code formatter on this site clearly doesn’t recognise
// as a comment marker…)
If there’s nothing useful you could say there, then there’s probably no need for a comment; an empty block is already pretty clear.
Also, I think the
= Unit version might be a little harder to follow than the others, especially for those less experienced in the language, as it’s so different from the normal block bodies.⠀(And even if you’re used to expression bodies, it’s pretty rare to need to refer to the
Unit value explicitly.)