[BUG] not nullable property is shown as nullable

Created by: whirm

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

The minimal declaration file pasted below results in the Bar.notNullable property getting isNullable: "true" set.

openapi-generator version

Current master (40018c33)

It's a regresssion.

OpenAPI declaration file content or url

---
openapi: 3.0.3
info:
  title: Foo
  version: v1
servers:
- url: /
  description: Foo
paths:
  /foo:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Bar'
      responses:
        "200":
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Bar'
components:
  schemas:
    Bar:
      required:
      - foo
      type: object
      properties:
        notNullable:
          allOf:
          - $ref: '#/components/schemas/EnumFoo'
            nullable: false
    EnumFoo:
      enum:
      - FOO
      - BAR
      type: string

Generation Details

Steps to reproduce

 openapi-generator-cli generate -g rust -o out -i openapi.yml --global-property debugModels=true 

Cheers!