text: Title: Model property not optional when referencing a nullable subschema
  Body: **Describe the bug**
  
  Given an OpenAPI component schema configured with `nullable: true`, properties referencing this schema are not generated as optional.    
  
  **To Reproduce**
  
  Example schema (OpenAPI):
  ```yaml
  openapi: 3.0.3
  components:
    schemas:
      Parent:
        type: object
        required:
          - child
        properties:
          child:
            $ref: "#/components/schemas/Child"
      Child:
        nullable: true
        type: object
        required:
          - value
        properties:
          value:
            type: string
  ```
  
  Used commandline:
  ```sh
  datamodel-code-generator datamodel-codegen \
      --input ... \
      --input-file-type openapi \
      --output-model-type typing.TypedDict \
      --target-python-version 3.12 \
      --strict-nullable \
      --use-annotated \
      --use-field-description \
      --use-schema-description \
      --use-standard-collections \
      --use-union-operator \
      --disable-timestamp
  ```
  
  Model output
  ```py
  # generated by datamodel-codegen:
  #   filename:  openapi.yaml
  
  from __future__ import annotations
  
  from typing import TypedDict
  
  
  class Child(TypedDict):
      value: str
  
  
  class Parent(TypedDict):
      child: Child
  ```
  
  **Expected behavior**
  It is expected that the `child` property of the `Parent` model be optional.
  
  Model output
  ```py
  # generated by datamodel-codegen:
  #   filename:  openapi.yaml
  
  from __future__ import annotations
  
  from typing import TypedDict
  
  
  class Child(TypedDict):
      value: str
  
  
  class Parent(TypedDict):
      child: Child | None
  ```
  
  **Version:**
   - OS: iOS 15.1.1
   - Python version: 3.12.4
   - datamodel-code-generator version: 0.26.4
  