[BUG] The generated Ruby client's code violates its own Rubocop rule.

Created by: autopp

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?
[Optional] Bounty to sponsor the fix (example)

Description

The generated Ruby client's code violates its own Rubocop rule.

openapi-generator version

$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar version
4.0.0-SNAPSHOT

$ git log --oneline -n 1
0b10092262 (HEAD -> master, origin/master, origin/HEAD, autopp/master) [python-flask] [python-aiohttp] [bug] Fixed handling of dotted module names (#2041)

OpenAPI declaration file content or url

swagger: "2.0"
info:
  title: "Example API"
  version: "1.0.0"
host: "example.com"
paths:
  /foo:
    get:
      responses:
        200:
          description: ''

Command line used for generation

$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate --skip-validate-spec -g ruby -o /tmp/gem -i 'https://gist.githubusercontent.com/autopp/86b54a836b45332da307b2a998d9cd1f/raw/2d4d7ce07450a5a99d9dc32bc12449a9eac5afce/example.yml'

Steps to reproduce

Run the above command to generate client code for Ruby.
Go to /tmp/gem and run rubocop to check style of the generated model code.

Get warning in lib/openapi_client/api/default_api.rb:

$ (cd /tmp/gem; rubocop)
Inspecting 13 files
........C....

Offenses:

lib/openapi_client/api/default_api.rb:61:1: C: Layout/EmptyLinesAroundClassBody: Extra empty line detected at class body end.

13 files inspected, 1 offense detected

Related issues/PRs

N/A

Suggest a fix

Just fix this: https://github.com/OpenAPITools/openapi-generator/blob/0b1009226276a4869cc3b9dc6a17cd0b143741a9/modules/openapi-generator/src/main/resources/ruby-client/api.mustache#L191