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

### Problem
When doing `render nothing: true`, it should render a body == ""

### Cases:
`render nothing: true` body should be ""
`render nothing: true, body: nil` body should be " "
`render nothing: true, text: nil` body should be " "

As far as I can see the history, seems like this problem was introduces 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.