[BUG][GO] imported and not used compilation error

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

If a controller have all its operations without Path Param or Body Param, generated code has unused import and compilation will fail.

openapi_gen/api_user.go:13:2: imported and not used: "encoding/json"
openapi_gen/api_user.go:17:2: imported and not used: "github.com/gorilla/mux"

openapi-generator version

docker openapitools/openapi-generator-cli:v4.2.2

OpenAPI declaration file content or url

  /one/endpoint:
    get:
      tags:
        - one
      summary: summary
      description: description
      operationId: getInfo
      responses:
        '204':
          description: successful operation

Command line used for generation

docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli:v4.2.2 generate -i /local/specs.yaml -p sourceFolder=openapi_gen  -g go-server -o /local

Steps to reproduce

1. Generate code
2. build the generated code

Related issues/PRs

Suggest a fix

On my side, I fixed the issue by running goimports after the generation of the code. Maybe we could run something similar directly inside openapi-generator-cli.

Reproduce with the Docker v4.2.2 command and the minimal OpenAPI declaration, then build the generated go-server output. Start with openapi_gen/api_user.go and the Go server generator entry point; done means a controller with no path or body parameters builds without unused-import errors.