[elm] Fix operations with empty responses

[eriktim]

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: Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Operations without response result in invalid Elm code. This PR fixes that.

[wing328]

@trenneman thanks for the PR. Can you please update the Elm petstore sample so that the CI can verify the change?

[eriktim]

I ran the script ./bin/elm-petstore.sh. The Petstore example isn't affected by this bug, so nothing changed.

[wing328]

@trenneman 👌 PR merged into maser 👍