[BUG][Elm] Invalid syntax for array type query params

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

Openapi client generator for Elm generates invalid syntax for array type query params.

Output:

getArticles : Maybe List ArticleCategory -> Api.Request ()
getArticles category_query =
    Api.request
        "GET"
        "/articles"
        []
        [ ( "category", Maybe.map String.join "," << List.map  category_query ) ]
        []
        Nothing
        (Json.Decode.succeed ())

Expected output:

getArticles : Maybe (List ArticleCategory) -> Api.Request ()
getArticles category_query =
    Api.request
        "GET"
        "/articles"
        []
        [ ( "category", Maybe.map String.join "," << List.map  category_query ) ]
        []
        Nothing
        (Json.Decode.succeed ())

openapi-generator version

openapi-generator-cli-5.0.0-20201104.185152-853.jar

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: 0.1.0-beta
  title: Foo API

paths:
  /articles:
    get:
      summary: Get articles
      operationId: GetArticles
      parameters:
        - name: category
          in: query
          description: Comma separated list of categories to filter
          required: false
          schema:
            # NOTE elm code generation for array type query params is buggy as of now.
           type: array
           items:
              $ref: "#/components/schemas/articleCategory"
      responses:
        "200":
          description: Articles
components:
  schemas:
    articleCategory:
      type: string
      enum:
        - Travel
        - Business

Generation Details

java -jar ~/Downloads/openapi-generator-cli-5.0.0-20201104.185152-853.jar generate -i openapi.yaml -g elm -o elm-client

Steps to reproduce

• Write openapi.yaml.
• Generate the elm client code.

Related issues/PRs

Suggest a fix

Start with the supplied openapi.yaml and reproduce the issue using the documented java -jar ... generate -i openapi.yaml -g elm command. Inspect the Elm generator's handling of optional array query parameters; done means the generated function type uses valid Elm syntax, including parentheses around List ArticleCategory, while preserving the request behavior.