[BUG][JAVA][Gradle Plugin] Java client generator not initializing fields

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

Using the openapi-generator gradle plugin I setup a task to generate code for the Petstore example YAML. Checking the generated Java classes I can see this in Pet.java:

 @JsonProperty("id")
  private Long id = ;

  @JsonProperty("name")
  private String name = ;

  @JsonProperty("tag")
  private String tag = ;

My Gradle task:

task generatePetStore(type: org.openapitools.generator.gradle.plugin.tasks.GenerateTask) {
    String basePackage = "com.example.petstore"
    verbose = true
    library = "resttemplate"
    generatorName = "java"
    inputSpec = "${project.rootDir}/swagger/petstore.yaml".toString()
    outputDir = "${project.buildDir}/generated/api/petstore"
    invokerPackage = basePackage
    apiPackage = basePackage + ".controller"
    modelPackage = basePackage + ".model"
    configOptions = [
            dateLibrary: "java8-localdatetime"
    ]
}

openapi-generator version

openapi-generator-gradle-plugin 4.2.2

OpenAPI declaration file content or url

URL: https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/petstore.yaml

Command line used for generation

Steps to reproduce

1. Download Petstore Example YAML
2. Run code generation using the Gradle Plugin
3. Navigate to the Pet.java generated file

Related issues/PRs

Suggest a fix

The fields probably need to be initialized to null at least as the generated code is invalid Java syntax. Something like

  @JsonProperty("id")
  private Long id = null;

  @JsonProperty("name")
  private String name = null;

  @JsonProperty("tag")
  private String tag = null;

Reproduce the issue with the Petstore YAML and the Gradle GenerateTask configuration, then inspect the generated Pet.java model. Trace the Java generator template or entry point responsible for model field declarations. Done means the generated Java source is valid and fields are initialized consistently with the reported expected output.