[BUG][csharp-nancyfx] An invalid c# Modules file is created under certain conditions

Created by: tzickel

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?

Description

Sometimes (I'm not sure what's the trigger), the c# nancyfx code generator emits an empty (and invalid) "using ;") line in a Modules file.

openapi-generator version

master

OpenAPI declaration file content or url

swagger: "2.0"
info:
  version: "0.0.1"
  title: "bug"
host: "localhost:8080"
basePath: "/v1alpha1"
schemes:
- "http"
paths:
  /test:
    post:
      parameters:
      - in: "body"
        name: "body"
        schema:
          type: "object"
          properties:
            name:
              type: "string"
      responses:
        "204":
          description: "successful operation"

Generation Details

standard csharp-nancyfx generation.

Steps to reproduce

generate the above spec file and look in the file: src/Org.OpenAPITools/Modules/DefaultModule.cs