[Erlang] return empty body for 204

[lixen-wg2]

cowboy_req:reply does not allow non empty body for 204.
https://github.com/ninenines/cowboy/blob/master/src/cowboy_req.erl#L807

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[wing328]

cc @tsloughter (2017/11) @jfacorro (2018/10) @robertoaloi (2018/10)

[wing328]

Tested locally by updating the samples and the output compiles without issues.

I'll update the samples in another commit after merging this PR.