Describe the bug
According to this OpenAPI guide, there are two ways to define free-form objects (a.k.a., a dictionary with values of any type).
They are equivalent and we expect the code generator to produce the same results Optional[Dict[str, Any]] = None.
Unfortunately only one of the syntaxes works as expected.

To Reproduce

Example schema with syntax 1 (additionalProperties: true)

components:
  schemas:
    CustomObject:
        type: object
        properties:
          config:
            $ref: '#/components/schemas/Config'
    Config:
      type: object
      additionalProperties: true

Output of syntax 1

class Config(BaseModel):
    pass
    model_config = ConfigDict(
        extra="allow",
    )


class CustomObject(BaseModel):
    config: Optional[Config] = None

Example schema with syntax 2 (additionalProperties: {})

components:
  schemas:
    CustomObject:
        type: object
        properties:
          config:
            $ref: '#/components/schemas/Config'
    Config:
      type: object
      additionalProperties: {}

Output of syntax 2

class CustomObject(BaseModel):
    config: Optional[Dict[str, Any]] = None

Used commandline:

$ datamodel-codegen --input test.yaml --input-file-type openapi --output test.py --snake-case-field --target-python-version 3.9 --use-schema-description --field-constraints --use-annotated --collapse-root-models --use-one-literal-as-default --enum-field-as-literal one --output-model-type pydantic_v2.BaseModel

Expected behavior
We expect both syntaxes will result in Output of syntax 2.

Version:

• OS: macOS Ventura 13.6.1
• Python version: 3.9.18
• datamodel-code-generator version: 0.22.1, 0.25.1