content-legth should be eq 0 when render nothing: true

### Problem
When doing `render nothing: true`, it should render a body == ""

This how Rails behaves since 2009, because of a Safari bug.

~~As far as I can see the history, seems like this problem was introduced by this commit https://github.com/rails/rails/commit/53596d091390bda7fb9b78ad838273a9e63fbde1 . Before that change if you only pass a `nothing: true` body would be nil.~~

[fixes #14336]

review @rafaelfranca @carlosantoniodasilva @sikachu

~~I am not sure about the implementation. looks dirty, but I couldnt think a better way. I am opened for suggestions.~~


If it is all good, I can add a CHANGELOG.