[BUG] [Elm] String Type schemas cause the Data.elm to not be generated

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The elm generator doesn't create a Data.elm in src/Api/ which leads to non-compiling code if the api spec contains a schema which is simply a string (see provided gist).

openapi-generator version

openapi-generator-cli 5.2.0
  commit : 90f7bcd
  built  : 2021-07-09T09:41:46Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

Not sure if this worked at some point.

OpenAPI declaration file content or url

A minimal example can be seen here: https://gist.github.com/alehed/e5eb44e14f0dcf38a430421e62f9aa12

Generation Details

openapi-generator-cli generate -i https://gist.githubusercontent.com/alehed/e5eb44e14f0dcf38a430421e62f9aa12/raw/e57d418eeebc8f3553a815522401047c7cebec5a/swapi-dummy.yaml -g elm -o generated

Steps to reproduce

Run the above command and inspect the generated folder.

Run the provided openapi-generator-cli command with the linked minimal specification, then inspect the generated folder and the Elm generator's handling of string-only schemas. Confirm the missing src/Api/Data.elm and trace why dependent generated code is not compilable. Done means the same specification generates Data.elm and the resulting Elm project compiles.