[BUG] [java] non-required arrays fail validation

Created by: ashatch

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

Optional array properties in JSON responses fail validation when an optional array property is null:

      // ensure the json data is an array
      if (!jsonObj.get("anotherThing").isJsonArray()) {
        throw new IllegalArgumentException(String.format("Expected the field `linkedContent` to be an array in the JSON string but got `%s`", jsonObj.get("linkedContent").toString()));
      }

A null check is required.

java.lang.NullPointerException: Cannot invoke "com.google.gson.JsonElement.isJsonArray()" because the return value of "com.google.gson.JsonObject.get(String)" is null

Expected to check null and validate successfully.

openapi-generator version

Current master (0d56ebc).

OpenAPI declaration file content or url

Declaration of an array property:

components:
  schemas:
    SomeObject:
      properties:
        something:
          $ref: '#/components/schemas/Something'
        anotherThing:
          items:
            $ref: '#/components/schemas/OtherThingItem'
          type: array

Suggest a fix

Looks like it was introduced in 3ae37e27, pojo.moustache line 500. I'll work on a PR.