[BUG][PYTHON] Cannot generate required but nullable object in 7.3.0

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

In the OpenApi, propertyA is both required and Nullable, in 7.2.0 this was declared as Optional

class ObjectDto(BaseModel):
    """
    ObjectDto
    """ # noqa: E501
    property_a: Optional[SubObject] = Field(alias="propertyA")

but in 7.3.0 the Optional disappeared

class ObjectDto(BaseModel):
    """
    ObjectDto
    """ # noqa: E501
    property_a: ObjectDtoPropertyA = Field(alias="propertyA")

Which means from_dict is failing despite null being valid

        _obj = cls.model_validate({
            "propertyA": ObjectDtoPropertyA.from_dict(obj["propertyA"]) if obj.get("propertyA") is not None else None
        })

openapi-generator version

Regression between 7.2.0 and 7.3.0

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: API
  version: '0.1'

paths:
  /api/postA:
    post:
      responses:
        '200':
          description: success
          content:
            application/json:
              schema:
                allOf:
                  - $ref: '#/components/schemas/ObjectDto'
components:
  schemas:
    ObjectDto:
      properties:
        propertyA:
          anyOf:
            - $ref: '#/components/schemas/SubObject'
            - type: 'null'
      type: object
      title: ObjectDto
      required:
        - propertyA
    SubObject:
      type: string

Generation Details

Simple generation in 7.3.0 with -g python

Steps to reproduce

ObjectDto(property_a=None)

pydantic_core._pydantic_core.ValidationError: 1 validation error for ObjectDto
property_a
  Input should be a valid dictionary or instance of ObjectDtoPropertyA [type=model_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.5/v/model_type

Related issues/PRs

Seems to be the same problem as #17938 in java

Suggest a fix

I see a lot of things were moved around so I am not sure about a fix

Start by generating the supplied OpenAPI 3.1 specification with the Python generator using 7.2.0 and 7.3.0, then compare the generated ObjectDto model and its from_dict behavior. The fix is complete when a required nullable property remains nullable and ObjectDto(property_a=None) validates successfully.