[BUG] additionalProperties with nullable generates broken model

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

Using "additionalProperties: true" together with "nullable: true" on "type: object" or "type: array" generates a broken model class which can not compile:

incompatible types: cannot infer type arguments for java.util.HashMap<>
cannot find symbol

Because it uses an Map together with JsonNullable:

@JsonProperty("foobar")
@Valid
private JsonNullable<Map<String, Object>> foobar = JsonNullable.undefined();

But generates an put method without JsonNullable:

  public Pet putFoobarItem(String key, Object foobarItem) {
    if (this.foobar == null) {
      this.foobar = new HashMap<>();
    }
    this.foobar.put(key, foobarItem);
    return this;
  }

openapi-generator version

5.3.1

OpenAPI declaration file content or url

components:
  schemas:
    Pet:
      type: object
      required:
        - id
        - name
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
        foobar:
          type: object
          additionalProperties: true
          nullable: true

https://github.com/noshua/openapi-additionalPropertiesWithNullable/blob/master/src/main/resources/petstore.yml

Generation Details

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>5.3.1</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>
                                ${project.basedir}/src/main/resources/petstore.yml
                            </inputSpec>
                            <generatorName>spring</generatorName>
                            <apiPackage>com.baeldung.openapi.api</apiPackage>
                            <modelPackage>com.baeldung.openapi.model</modelPackage>
                            <supportingFilesToGenerate>
                                ApiUtil.java
                            </supportingFilesToGenerate>
                            <configOptions>
                                <delegatePattern>true</delegatePattern>
                                <sourceFolder>src/gen/java/main</sourceFolder>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

https://github.com/noshua/openapi-additionalPropertiesWithNullable.git

Related issues/PRs

Suggest a fix

Generated put method should look something like this:

  public NosJobShort putExtraOptsItem(String key, Object extraOptsItem) {
    if (!this.extraOpts.isPresent()) {
      this.extraOpts = JsonNullable.of(new HashMap<>());
    }
    this.extraOpts.get().put(key, extraOptsItem);
    return this;
  }

Start with the linked reproduction repository and generate the Spring model from its petstore.yml using the stated openapi-generator Maven configuration. Compare the nullable additionalProperties field with its generated put method; done means the generated Java model compiles and handles the nullable map correctly.