[BUG][JAVA] Syntax error generating JAVA class from params object with certain schema

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

Generator generates broken (meaning it contains syntax error) class when an endpoint parameter schema object

1. has a property which is also an object and
2. that property object has both some properties and additionalProperties specified of same type.

Please note "PostParams" schema below.

openapi-generator version

5.1.1, 5.2.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Example API
  version: 0.0.1
servers:
  - url: http://example.com
paths:
  /content:
    post:
      summary: Fetch 'something' using a POST request
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/PostParams'
      responses:
        200:
          description: The fetched 'something'
          content:
            application/json:
              schema:
                type: object

components:
  schemas:
    PostParams:
      type: object
      properties:
        mapOfStringLists:
          type: object
          description: ARBITRARY map of string lists.
          additionalProperties:
            type: array
            items:
              type: string
          properties:
            property1:
              type: array
              items:
                type: string

Generation Details

Running mvn build generates the following function (improper!!!) in PostParams.java:

  public PostParams putMapOfStringListsItem(String key, List<String> mapOfStringListsItem) {
    if (this.mapOfStringLists == null) {
      this.mapOfStringLists = ; // NOTE THE MISSING ASSIGNMENT HERE!!!
    }
    this.mapOfStringLists.put(key, mapOfStringListsItem);
    return this;
  }

Steps to reproduce

Run mvn build having the following in pom.xml:

    <build>
        <plugins>
            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>${openapi-generator.version}</version>
                <executions>
                    <execution>
                        <id>someid</id>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>path/to/swagger.yaml</inputSpec>
                            <generatorName>java</generatorName>
                            <library>rest-assured</library>
                            <output>target/openapi</output>
                            <apiPackage>some.package.name</apiPackage>
                            <modelPackage>some.model.name</modelPackage>
                            <configOptions>
                                <interfaceOnly>true</interfaceOnly>
                                <sourceFolder>src/gen/java</sourceFolder>
                                <dateLibrary>java8</dateLibrary>
                                <serializationLibrary>jackson</serializationLibrary>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

Related issues/PRs

My search criteria was: https://github.com/OpenAPITools/openapi-generator/issues?q=is%3Aissue+is%3Aopen+in%3Atitle+%5BJAVA%5D+additionalProperties+
Most relevant issue (in my read) which is only similar (only) is: https://github.com/OpenAPITools/openapi-generator/issues/9791

Suggest a fix

I cannot suggest a fix unfortunately.

Reproduce the issue with the provided OpenAPI declaration and Maven configuration, then inspect the generated PostParams.java for the missing assignment. Trace the Java generator handling of a nested object that combines properties and additionalProperties of the same type, and add regression coverage showing that the generated class compiles with the expected assignment.