[BUG] [JAVA] 204 should be a valid response for `text/plain`

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• [] 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

I have the following endpoint public String getGlobalConfigurationValueByKey(@QueryParam("key") String key)
which returns a String with Content-Type text/plain as an HTTP 200 response.

But if there is no such entry, it returns HTTP 204 and does not add a Content-Type header.

The generated code, however, can't cope with this, complaining:

Error! The response Content-Type is supposed to be text/plain but it's not: (GET https://someh/api/v1/config/global?key=mandanten%2Ffarb) 204

which is located in

https://github.com/OpenAPITools/openapi-generator/blob/ddb15d4b9d6fa0f04090b5db3d64ea1b8a6d0ee6/modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache#L449

openapi-generator version

			<groupId>org.openapitools</groupId>
			<artifactId>openapi-generator-maven-plugin</artifactId>
			<!-- RELEASE_VERSION -->
			<version>7.16.0</version>

OpenAPI declaration file content or url

 /api/v1/config/global:
    put:
      operationId: setGlobalConfigurationValueByKey
      tags:
      - user
      - config
      parameters:
      - name: key
        in: query
        schema:
          type: string
      requestBody:
        content:
          text/plain:
            schema:
              type: string
        required: true
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type: string
        "401":
          description: Not Authorized
        "403":
          description: Not Allowed
      summary: Set Global Configuration Value By Key
      security:
      - ElexisEnvironment:
        - api-access
        - user
      description: "🔓 **Roles Required:** api-access,user<br>"
    get:
      operationId: getGlobalConfigurationValueByKey
      tags:
      - user
      - config
      parameters:
      - name: key
        in: query
        schema:
          type: string
      responses:
        "200":
          description: OK
          content:
            text/plain:
              schema:
                type:
                - string
        "401":
          description: Not Authorized
        "403":
          description: Not Allowed
      summary: Get Global Configuration Value By Key
      security:
      - ElexisEnvironment:
        - api-access
        - user
      description: "🔓 **Roles Required:** api-access,user<br>"

Suggest a fix

I propose accepting a 204, and adding the following code wich seems legit, and works for me as a solution.

        if(204==localVarResponse.statusCode()) {
        	return new ApiResponse<String>(
                    localVarResponse.statusCode(),
                    localVarResponse.headers().map(),
                    null
            );
        }

If anyone is willing to take it, I'll prepara a pull request.

Start at modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache around line 449, using the provided OpenAPI declaration and generated Java client scenario. Verify the behavior for a 204 response with text/plain, then add coverage for the case and confirm the generated client accepts the response and returns the expected empty value.