[Kotlin] Empty POST body causes KotlinNullPointerException

Description

Swagger-codegen version

2.2.3

Swagger declaration file content or url

https://gist.github.com/NikitoZZZZZ/7d3340b6668a269ad8e8dfe2189abc8e

Command line used for generation

https://gist.github.com/NikitoZZZZZ/6303480bffdfa670db124878bc9e1633

Steps to reproduce

Generated method from my swagger api doc looks like this:

@Suppress("UNCHECKED_CAST")
    fun cancelNotificationUsingPOST(id: kotlin.String) : NotificationResponse {
        val localVariableBody: kotlin.Any? = null
        val localVariableQuery: MultiValueMap = mapOf()
        val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf()
        val localVariableConfig = RequestConfig(
            RequestMethod.POST,
            "/api/v1/notification/{id}/cancel".replace("{"+"id"+"}", "$id"),
            query = localVariableQuery,
            headers = localVariableHeaders
        )
        val response = request<NotificationResponse>(
            localVariableConfig,
            localVariableBody
        )

        return when (response.responseType) {
            ResponseType.Success -> (response as Success<*>).data as NotificationResponse
            ResponseType.Informational -> TODO()
            ResponseType.Redirection -> TODO()
            ResponseType.ClientError -> throw ClientException((response as ClientError<*>).body as? String ?: "Client error")
            ResponseType.ServerError -> throw ServerException((response as ServerError<*>).message ?: "Server error")
            else -> throw kotlin.IllegalStateException("Undefined ResponseType.")
        }
    }

Generated ApiClient class has method request that looks like this:

inline protected fun <reified T: Any?> request(requestConfig: RequestConfig, body : Any? = null): ApiInfrastructureResponse<T?> {
        val httpUrl = HttpUrl.parse(baseUrl) ?: throw IllegalStateException("baseUrl is invalid.")

        var urlBuilder = httpUrl.newBuilder()
                .addPathSegments(requestConfig.path.trimStart('/'))

        requestConfig.query.forEach { k, v ->
            v.forEach { queryValue ->
                urlBuilder = urlBuilder.addQueryParameter(k,queryValue)
            }
        }

        val url = urlBuilder.build()
        val headers = requestConfig.headers + defaultHeaders

        if(headers[ContentType] ?: "" == "") {
            throw kotlin.IllegalStateException("Missing Content-Type header. This is required.")
        }

        if(headers[Accept] ?: "" == "") {
            throw kotlin.IllegalStateException("Missing Accept header. This is required.")
        }

        // TODO: support multiple contentType,accept options here.
        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
        val accept = (headers[Accept] as String).substringBefore(";").toLowerCase()

        var request : Request.Builder =  when (requestConfig.method) {
            RequestMethod.DELETE -> Request.Builder().url(url).delete()
            RequestMethod.GET -> Request.Builder().url(url)
            RequestMethod.HEAD -> Request.Builder().url(url).head()
            RequestMethod.PATCH -> Request.Builder().url(url).patch(requestBody(body!!, contentType))
            RequestMethod.PUT -> Request.Builder().url(url).put(requestBody(body!!, contentType))
            RequestMethod.POST -> Request.Builder().url(url).post(requestBody(body!!, contentType))
            RequestMethod.OPTIONS -> Request.Builder().url(url).method("OPTIONS", null)
        }

        headers.forEach { header -> request = request.addHeader(header.key, header.value) }

        val realRequest = request.build()
        val response = client.newCall(realRequest).execute()

        // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
        when {
            response.isRedirect -> return Redirection(
                    response.code(),
                    response.headers().toMultimap()
            )
            response.isInformational -> return Informational(
                    response.message(),
                    response.code(),
                    response.headers().toMultimap()
            )
            response.isSuccessful -> return Success(
                    responseBody(response.body(), accept),
                    response.code(),
                    response.headers().toMultimap()
            )
            response.isClientError -> return ClientError(
                    response.body()?.string(),
                    response.code(),
                    response.headers().toMultimap()
            )
            else -> return ServerError(
                    null,
                    response.body()?.string(),
                    response.code(),
                    response.headers().toMultimap()
            )
        }
    }

As you can see variable localVariableBody is set to null in method cancelNotificationUsingPOST because POST method doesn't have a body. So I get KotlinNullPointerException in method request in line RequestMethod.POST -> Request.Builder().url(url).post(requestBody(body!!, contentType)) since parameter body!! is null.

Suggest a fix/enhancement

Change POST body generation from null to empty string(""). WA for now is to set it to "" manually.

Start by reproducing the generated cancelNotificationUsingPOST call from the linked command, then inspect the generated ApiClient.request entry point and its POST handling. Done means a POST operation without a declared body no longer throws KotlinNullPointerException and still produces a valid request.