[Java] No Import Statement generated for Nested additionalProperties field

Description

I have a return type for a 200 response code that is a nested "additionalProperties" field that generates to Map<String, Map<String, Link>>. Within the generated SDK code, the generator omits the import for the Link class. It omits it for both the API class (WebMvcLinksHandlerApi.java) as well as the associated test class (WebMvcLinksHandlerApiTest.java). As a result, the Java code does not compile without manually adding the import.

Swagger-codegen version

3.0.10

I have also tried other version and it failed in those too.

Swagger declaration file content or url

Nested additional Properties

"additionalProperties":{
   "type":"object",
   "additionalProperties":{
      "$ref":"#/definitions/Link"
   }
}

Nested additional Properties with more JSON context

"/actuator":{
      "get":{
        "tags":[
          "web-mvc-links-handler"
        ],
        "summary":"links",
        "operationId":"linksUsingGET",
        "produces":[
          "application/json",
          "application/vnd.spring-boot.actuator.v2+json"
        ],
        "parameters":[
          {
            "name":"x-vol-site",
            "in":"header",
            "required":false,
            "type":"integer",
            "format":"int32"
          },
          {
            "name":"x-vol-tenant",
            "in":"header",
            "required":true,
            "type":"integer",
            "format":"int32"
          }
        ],
        "responses":{
          "200":{
            "description":"OK",
            "schema":{
              "type":"object",
              "additionalProperties":{
                "type":"object",
                "additionalProperties":{
                  "$ref":"#/definitions/Link"
                }
              }
            }
          },
          "401":{
            "description":"Unauthorized"
          },
          "403":{
            "description":"Forbidden"
          },
          "404":{
            "description":"Not Found"
          }
        },
        "deprecated":false
      }
    }

Command line used for generation

swagger-codegen generate -i swagger.json -l java -c config.json -o tmp/

Steps to reproduce

Generate and compile via mvn clean install

Related issues/PRs

Maybe https://github.com/swagger-api/swagger-codegen/issues/3140 ?

Suggest a fix/enhancement

N/A

Start by reproducing the issue with the provided nested additionalProperties schema and the swagger-codegen generate command, then run mvn clean install. Inspect generation of WebMvcLinksHandlerApi.java and WebMvcLinksHandlerApiTest.java; done means both compile with the required Link import generated automatically.