[JAVA SPRING] Null pointer exception when specification doesn't have components

Description

when the open API specification doesn't contain components then null pointer exception is thrown while generating models.
Error:
Exception in thread "main" java.lang.NullPointerException
at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:334)
at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:779)
at org.specocode.codegen.CodeGenerator.generateServerStub(CodeGenerator.java:73)
at org.specocode.codegen.CodeGenerator.main(CodeGenerator.java:207)

Swagger-codegen version

3.0.20

Swagger declaration file content or url

openapi: 3.0.1
info:
  title: Swagger Isuite
  description: '<p>This is a sample server Petstore server. You can find out more
    about Swagger at [http://swagger.io](http://swagger.io) or on [irc.freenode.net,
    #swagger](http://swagger.io/irc/). For this sample, you can use the api key `special-key`
    to test the authorization filters.</p>'
  version: 1.0.0
servers:
- url: https://api.pethub.com/pethub
- url: http://api.pethub.com/pethub
paths:
  /store:
    description: store details
    get:
      description: get all stores
      responses:
        200:
          description: store information details
          content:
            'application/json':
              schema:
                type: object
                properties:
                  id:
                    type: string
                  name:
                    type: string
                  location:
                    type: string
                  pincode:
                    type: number

Steps to reproduce

1. Parse json and get OpenAPI object
2. Config used for codegen is as below,
   CodegenConfig config = CodegenConfigLoader.forName("spring");
   ((SpringCodegen) config).setApiPackage("api");
   ((SpringCodegen) config).setArtifactId("org.mycode.pet");
   ((SpringCodegen) config).setBasePackage("org.mycode");
   ((SpringCodegen) config).setArtifactVersion("1.0.0");
   ((SpringCodegen) config).setModelPackage("model");
   ((SpringCodegen) config).setConfigPackage("config");
   ((SpringCodegen) config).setGroupId("org.specocode");
   ((SpringCodegen) config).setJava8(true);
   ((SpringCodegen) config).setLibrary("spring-boot");
   config.additionalProperties().put("basePackage", "org.specocode");
   config.setOutputDir("someOutputfolder");
3. ClientOptInput is created
   ClientOptInput opts = new ClientOptInput();
   opts.setOpenAPI(openAPI);
   opts.setConfig(config);
4. DefaultGenerator's generate method is called
   DefaultGenerator generator = new DefaultGenerator();
   List files = generator.opts(opts).generate();

Start in DefaultGenerator.java at generateModels line 334, then follow the generate call shown in the stack trace. Reproduce the failure with the supplied OpenAPI YAML, which has paths but no components. Done means generation completes without a NullPointerException for specifications without components, with regression coverage for that input.