[BUG][kotlin][jvm-retrofit2] unnecessary nullable type of OkHttpClient / small fix in docstring

Created by: warahiko

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

1. unnecessary nullable type of OkHttpClient

In the function createService() of generated infrastructure/ApiClient.kt file, the variable usedClient has nullable type. But here it should be NonNullable.

Retrofit.Builder.client() receives OkHttpClient instance, and checks it is NonNull.

So in IDE such as AndroidStudio, an error occurs.

The actual output

fun <S> createService(serviceClass: Class<S>): S {
    var usedClient: OkHttpClient? = null
    this.okHttpClient?.let { usedClient = it } ?: run {usedClient = clientBuilder.build()}
    return retrofitBuilder.client(usedClient).build().create(serviceClass)
}

The expected output

fun <S> createService(serviceClass: Class<S>): S {
    val usedClient: OkHttpClient = this.okHttpClient ?: clientBuilder.build()
    return retrofitBuilder.client(usedClient).build().create(serviceClass)
}

2. small fix in docstrings

In the generated API files, the indent of the docstrings is irregular.

The actual output

interface PetApi {
    /**
     * Add a new pet to the store
     * 
     * Responses:
     *  - 405: Invalid input
     * 
     * @param body Pet object that needs to be added to the store 
    * @return [Call]<[Unit]>
     */
    @POST("pet")
    fun addPet(@Body body: Pet): Call<Unit>

...
}

The expected output

interface PetApi {
    /**
     * Add a new pet to the store
     * 
     * Responses:
     *  - 405: Invalid input
     * 
     * @param body Pet object that needs to be added to the store 
     * @return [Call]<[Unit]>
     */
    @POST("pet")
    fun addPet(@Body body: Pet): Call<Unit>

...
}

openapi-generator version

master (61b543f0)

OpenAPI declaration file content or url

https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml

(it is described in 2 - Getting Started)

Generation Details

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate \
-i https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml \
-g kotlin --library jvm-retrofit2 -o kotlin-retrofit2-client -t modules/openapi-generator/src/main/resources/kotlin-client

Steps to reproduce

Run the above command-line

Related issues/PRs

Suggest a fix

I will submit a PR.