[BUG] [Kotlin] Optional parameters

Bug Report Checklist

• [+] Have you provided a full/minimal spec to reproduce the issue?
• [+] Have you validated the input using an OpenAPI validator (example)?
• [+] What's the version of OpenAPI Generator used?
• [+] Have you search for related issues/PRs?
• [+] What's the actual output vs expected output?
  Generated argument types are kotlin.String, but should be kotlin.String?

Description

Client code generated: It is impossible to not provide an optional param to an API.

openapi-generator version

4.0.0-SNAPSHOT

OpenAPI declaration file content or url

swagger: "2.0"
info:
  version: "2.1.4"
  title: "Test"
  description: ""
host: "example.cm"
basePath: "/2_0"
schemes:
  - "https"
paths:
  /data/foo:
    get:
      operationId: "getFoo"
      produces:
        - "application/json"
      parameters:
        - name: "t0"
          in: "query"
          required: false
          type: "string"
          format: "date-time"
          description: "Filter by start time, e.g. '2018-03-01T00-00'"
      responses:
        200:
          description: "200 response"
          schema:
            type: "string"

Command line used for generation

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i test.yaml -g kotlin -o tmp/kotlin/

Steps to reproduce

1. Generate the code.
2. Open vi tmp/kotlin/src/main/kotlin/org/openapitools/client/apis/DefaultApi.kt
3. See getFoo argument type.

Suggest a fix

Arguments for which spec says "required: false", should be optional (followed by ? in Kotlin).

Reproduce the issue with the supplied OpenAPI YAML and the documented generation command, then inspect tmp/kotlin/src/main/kotlin/org/openapitools/client/apis/DefaultApi.kt at getFoo. Trace how the Kotlin generator derives that argument type; done means an optional parameter is generated with the nullable kotlin.String? type and the generated client allows it to be omitted.