[Bug] Java Syntax Error for additionalProperties

Description

For object schemas with additionalProperties the java generator produces code with a syntax error:

public MultiPart putHeadersItem(String key, List<String> headersItem) {
    if (this.headers == null) {
      this.headers = ; // <- HERE IS THE SYNTAX ERROR
    }
    this.headers.put(key, headersItem);
    return this;
  }

openapi-generator version

I using the maven plugin openapi-generator-maven-plugin in version 5.0.0-beta3

OpenAPI declaration file content or url

bug-api.yaml to reproduce the problem:

---
openapi: 3.0.1
servers:
  - url: "/rest/1.0"
info:
  title: Bug API
  version: "1.0"
paths:
  "/files":
    post:
      tags:
        - Files
      summary: Upload files
      description: Upload files
      operationId: addFiles
      requestBody:
        content:
          multipart/form-data:
            schema:
              "$ref": "#/components/schemas/MultiPart"
      responses:
        '201':
          description: Files successfully uploaded.
          content:
            application/json:
              schema:
                type: array
                items:
                  type: string
components:
  schemas:
    MultiPart:
      type: object
      properties:
        headers:
          type: object
          properties:
            empty:
              type: boolean
          additionalProperties:
            type: array
            items:
              type: string

pom.xml using the openapi-generator-maven-plugin in version 5.0.0-beta3 with java generator and native library:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>org.example</groupId>
  <artifactId>openapi-generator-bug</artifactId>
  <version>1.0-SNAPSHOT</version>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <!-- for openapi generator -->
    <dependency>
      <groupId>io.swagger</groupId>
      <artifactId>swagger-annotations</artifactId>
      <version>1.6.2</version>
    </dependency>
    <dependency>
      <groupId>com.google.code.findbugs</groupId>
      <artifactId>jsr305</artifactId>
      <version>3.0.2</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-annotations</artifactId>
      <version>2.12.0</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.datatype</groupId>
      <artifactId>jackson-datatype-jsr310</artifactId>
      <version>2.12.0</version>
    </dependency>
    <dependency>
      <groupId>org.openapitools</groupId>
      <artifactId>jackson-databind-nullable</artifactId>
      <version>0.2.1</version>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <!-- set Java 11 -->
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.1</version>
        <configuration>
          <release>11</release>
        </configuration>
      </plugin>

      <plugin>
        <groupId>org.openapitools</groupId>
        <artifactId>openapi-generator-maven-plugin</artifactId>
        <version>5.0.0-beta3</version>
        <executions>
          <execution>
            <id>generate-core-api</id>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              <inputSpec>${project.basedir}/bug-api.yaml</inputSpec>
              <generatorName>java</generatorName>
              <packageName>bug</packageName>
              <apiPackage>bug.api</apiPackage>
              <modelPackage>bug.model</modelPackage>
              <invokerPackage>bug</invokerPackage>
              <library>native</library>
              <configOptions>
                <sourceFolder>src/java</sourceFolder>
                <java8>true</java8>
                <dateLibrary>java8</dateLibrary>
              </configOptions>
              <generateApiTests>false</generateApiTests>
              <generateApiDocumentation>false</generateApiDocumentation>
              <generateModelTests>false</generateModelTests>
              <generateModelDocumentation>false</generateModelDocumentation>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

If you execute mvn clean compile, it generates the MultiPart.java in target/generated-sources/openapi/src/java/bug/model/MultiPart.java containing the syntax error mentioned above.

Command line used for generation

mvn clean compile

Suggest a fix/enhancement

I'm not into the mustache templates, but I guess fixing the template is probably enough.

Start with bug-api.yaml and run mvn clean compile to reproduce the generated target/generated-sources/openapi/src/java/bug/model/MultiPart.java output. Inspect the Java generator templates responsible for additionalProperties and verify that the generated MultiPart.java contains valid Java and compiles without the syntax error.