[JaxRS] compile error with spec without models

Created by: jmini

Description

When you generate a jersey server for a spec that have no model, you can not compile the generated project.

This might be true for other java generated project (java-client, spring?...)

Work-arround: add this section in your spec:

components:
  schemas:
    SomeObj:
      type: object
      properties:
        someProp:
          type: string

openapi-generator version

3.1.0-SNAPSHOT

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/001f5ae50d96e9b69e377f136b7fab20e8feba85/modules/openapi-generator/src/test/resources/3_0/ping.yaml

Command line used for generation

Core directly in a java program, with following option:

        JavaJerseyServerCodegen config = new JavaJerseyServerCodegen();
        config.setJava8Mode(true);
        config.setUseBeanValidation(true);

Steps to reproduce

Generate the project. run mvn verify in the generated project.

You get:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project ping-oapigen-jaxrs-jersey: Compilation failure: Compilation failure: 
[ERROR] ******/ping-oapigen-jaxrs-jersey/src/main/java/org/openapitools/api/impl/PingApiServiceImpl.java:[4,1] package org.openapitools.model does not exist
[ERROR] ******/ping-oapigen-jaxrs-jersey/src/gen/java/org/openapitools/api/PingApiService.java:[4,1] package org.openapitools.model does not exist
[ERROR] ******/ping-oapigen-jaxrs-jersey/src/gen/java/org/openapitools/api/PingApi.java:[3,1] package org.openapitools.model does not exist

Suggest a fix/enhancement

In the generated code, when there is no models, following import line should be removed:

import org.openapitools.model.*;