[BUG][Go][Client] Empty object generates compilation failure

Version
4.1.1

Command line used for generation
openapi-generator generate -i swagger.yaml -g go-o ./

Spec

swagger: "2.0"
info: 
  title: Foo
  version: v1
paths: 
  /foo: 
    post:
      operationId: fooPost
      parameters: 
        - 
          in: body
          name: body
          required: false
          schema: 
            $ref: "#/definitions/Foo"
      responses: 
        200: 
          description: "successful operation"
definitions: 
  Foo: 
    type: object

Description
Generated Foo model is:

type FooPostOpts struct {
	Body optional.Map[string]interface{}
}

which does not compile as optional.Map does not exist.
I would expect this model definition instead:

type FooPostOpts struct {
	Body map[string]interface{}
}

I can reproduce this issue with any object without properties.

cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)
@wing328

Start by running the provided openapi-generator command with the minimal swagger.yaml specification and inspect the generated Go client's FooPostOpts model. Trace how an empty object definition is represented for the optional body parameter, then verify that the generated client compiles and uses a valid map type.