[BUG] Endpoint returning imported type throws "attribute ... is missing" 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)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Generating for a declaration returning an imported type (type specified using importMappings) for an endpoint will throw an attribute ... is missing error.

openapi-generator version

5.0.1

OpenAPI declaration file

src/main/resources/test.yaml:

openapi: 3.0.3
info:
  title: test
  version: v1
paths:
  /test:
    get:
      responses:
        200:
          description: ok
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Test'
                #$ref: '#/components/schemas/TestAlias'
components:
  schemas:
    TestAlias:
      $ref: '#/components/schemas/Test'

Generation Details

build.gradle:

plugins { id "org.openapi.generator" version "5.0.1" }

apply plugin: 'java'
sourceCompatibility = '1.8'

openApiGenerate {
    generatorName = "spring"
    inputSpec = "${sourceSets.main.resources.srcDirs[0]}/test.yaml"
    configOptions = [interfaceOnly: "true"]
    //skipValidateSpec = true 
    importMappings = ["Test": "java.lang.String"]
}

Steps to reproduce

project structure:

src/main/resources/test.yaml
build.gradle

run: gradle openApiGenerate

Workaround till it's fixed

Referencing an alias type referring to the imported one (TestAlias the in example above) will make the generator work as expected.
Moreover running the generator for the original declaration (w/ Test) w/ skipValidateSpec = true the output will be exactly the same as running it for the alias type. (Of course the validation of other parts will also be skipped this way.)

Reproduce the failure with src/main/resources/test.yaml and build.gradle using the spring generator and gradle openApiGenerate. Compare generation for the imported Test type with the working TestAlias workaround, then trace the reported missing-attribute error. Done means the imported endpoint generates successfully without requiring skipValidateSpec while preserving the expected output.