I know zero go, despite having worked with it professionally for a short stint that I try and block out of my memory (not go's fault), anyway that first example looks like gibberish to me while the second is super clear.
Of course this is all guesswork based on similar stuff i've seen elsewhere, so i might be off, but i think despite not knowing the language it still looks readable if you know any other mainstream language (and exercise some guesswork/imagination :-P).
I can say the same about Fennel, List or Haskell.
The first example is the way to go in Go. It's idiomatic, not a hack. And it works the same way for many things (for example checking if a channel's closed)
you could write this in Go
if m[key] { }