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

[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 3ae37e2, pojo.moustache line 500. I'll work on a PR.

[Zomzog]

@wing328 I think this one can be closed with the merge of #13777 :)