[BUG][jaxrs-spec] Generated code is not valid with additionalProperties

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

Generated code is not valid. A schema that has additional properties generates

private @Valid LocalizedString name = new java.util.HashMap<>();

Which is not valid it should've been

private @Valid LocalizedString name = new LocalizedString();

openapi-generator version

4.0.3

OpenAPI declaration file content or url

    LocalizedString:
      description: |
        An object containing all the translations for a given object.  Each key represents an
        [IETF BCP 47](https://tools.ietf.org/html/bcp47) language tag.
      type: object
      title: Localized String
      properties:
        "und":
          description: Represents `und`etermined which is used as the default when a more specialized version is not available.
          type: string
      additionalProperties:
        x-additionalPropertiesName: language-tag
        description: |
          The key represents the [IETF BCP 47](https://tools.ietf.org/html/bcp47) language tag for clients to determine
           what string to display.
        type: string
      example:
        "und": "default locale"
        "en": "English"
        "en-CA": "Canadian English"
        "en-US": "US English"
        "zh": "Chinese"
        "zh-CN": "China Chinese (Simplified)"

Command line used for generation

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.0.3</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <configOptions>
                                <apiPackage>com.devhaus.dlm.api</apiPackage>
                                <booleanGetterPrefix>is</booleanGetterPrefix>
                                <dateLibrary>java8</dateLibrary>
                                <fullJavaUtil>true</fullJavaUtil>
                                <invokerPackage>com.devhaus.dlm.api</invokerPackage>
                                <java8>true</java8>
                                <modelPackage>com.devhaus.dlm.api.model</modelPackage>
                                <useSwaggerAnnotations>false</useSwaggerAnnotations>
                            </configOptions>
                            <generatorName>jaxrs-spec</generatorName>
                            <inputSpec>${project.basedir}/src/main/resources/openapi.yml</inputSpec>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/3388

Suggest a fix

Start by running the jaxrs-spec Maven generation with the provided OpenAPI YAML and inspect the generated LocalizedString model. Trace how additionalProperties determines the field initializer; done means the generated Java is valid and initializes the field as LocalizedString rather than HashMap.