[BUG][Haskell]Faulty generation for responses with no content

[mgajda]

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

All responses are assumed to have non-empty content, and generated Haskell code tries to parse the missing response body.
This makes responses with no body non-functional.

openapi-generator version

Git commit: 77e1ca56cf329bfb22b05f853587263fd0ff6503 (June 16th 2021)
Version in pom.xml is 5.2.0-SNAPSHOT.

OpenAPI declaration file content or url

Here is an example input spec:

openapi: 3.0.0
info:
  description: Minimal failing example
  version: "1.0.0"
  title: Minimal API
  contact:
    email: [email protected]

paths:
  /session:
    post:
      summary: start session
      operationId: startSession
      responses:
        '201':
          description: session created
       '404':
          description: service offline

Generation Details

This will generate API that declares JSON object on the output:

type MinimalAPI                                                                                                                                                           
    =    "session" :> Verb 'POST 200 '[JSON] () -- 'startSession' route                                                                                    
    :<|> Raw

This suggests that we have a content that conforms to InlineResponse201.
Attempt to use this endpoint when fail when trying to parse missing response body.

Steps to reproduce

Copy paste spec.yaml above and run:

openapi-generator-cli generate -i spec.yaml  -g haskell

Suggest a fix

This change of the generated code works:

type MinimalAPI                                                                                                                                                           
    =    "session" :> Verb 'POST 204 '[JSON] NoContent -- 'startSession' route                                                                                    
    :<|> Raw