[racket] Why does sync require at least one event?

I'm writing a macro that expands into a use of sync on a number of
async-channels. In the context of the macro, it makes sense to have it wait
on zero channels - it means effectively the same thing as "go to sleep
forever", or (sync never-evt).

However, sync requires it be passed at least one event. I can get around
this issue by just adding a never-evt to the argument list, but what's the
reason for the current behavior? It seems silly to disallow (sync) when
there's a seemingly obvious default behavior.
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