[Swift4] accept empty content with default client

[d-date]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

To fix #7850, we accept Void response as empty body. This behavior is same as PromiseKit and RxSwift response

Breaking Change

By merging this PR, for using default client users, they need to change response as follow warnings by compiler.

Before:

FakeAPI.deletePet(id: "1000") { (error) in
  if error == nil { // error will be tuple contain (response, error), so compiler will show `error == nil always true`
    // success
  }
}

After:

FakeAPI.deletePet(id: "1000") { (response, error) in
  if error == nil { 
    // still means success
  }

  if let _ = response {
   // this also means success
  }
}

or you can ignore response as below.

FakeAPI.deletePet(id: "1000") { (_, error) in
  if error == nil { 
    // still means success
  }
}

[wing328]

Thanks for the PR.

cc @jgavris @ehyche @Edubits @jaz-ah for review

[jgavris]

+1 We've actually been doing this, but didn't want to do a breaking change! What do the others think?

[jaz-ah]

@jgavris I think it's ok to add this as a breaking change to 3.0.0 which already has a lot of other breaking changes...