[PY] using Optional fields in pydantic schema raises an error with google_genai plugin

[pavelgj]

Describe the bug

Using the sample from getting started doc

class Recipe(BaseModel):
    title: str
    description: str
    prep_time: str
    cook_time: str
    servings: int
    ingredients: list[str]
    instructions: list[str]
    tips: Optional[list[str]]

# Generate structured recipe data using the same schema
result = await ai.generate(
    prompt=prompt,
    output_schema=Recipe,
)

Traceback (most recent call last):
  File ".../genkit-intro/lib/python3.13/site-packages/genkit/core/action/_action.py", line 519, in async_tracing_wrapper
    output = await afn(input, ctx)
             ^^^^^^^^^^^^^^^^^^^^^
  File ".../genkit-intro/lib/python3.13/site-packages/genkit/plugins/google_genai/models/gemini.py", line 683, in generate
    response = await self._generate(
               ^^^^^^^^^^^^^^^^^^^^^
        request_contents=request_contents, request_cfg=request_cfg, model_name=model_name
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File ".../genkit-intro/lib/python3.13/site-packages/genkit/plugins/google_genai/models/gemini.py", line 719, in _generate
    response = await self._client.aio.models.generate_content(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        model=model_name, contents=request_contents, config=request_cfg
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File ".../genkit-intro/lib/python3.13/site-packages/google/genai/models.py", line 8171, in generate_content
    response = await self._generate_content(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        model=model, contents=contents, config=parsed_config
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File ".../genkit-intro/lib/python3.13/site-packages/google/genai/models.py", line 6982, in _generate_content
    request_dict = _GenerateContentParameters_to_mldev(
        self._api_client, parameter_model
    )
  File ".../genkit-intro/lib/python3.13/site-packages/google/genai/models.py", line 852, in _GenerateContentParameters_to_mldev
    _GenerateContentConfig_to_mldev(
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        api_client, getv(from_object, ['config']), to_object
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ),
    ^
  File ".../genkit-intro/lib/python3.13/site-packages/google/genai/models.py", line 728, in _GenerateContentConfig_to_mldev
    t.t_schema(api_client, getv(from_object, ['response_schema'])),
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../genkit-intro/lib/python3.13/site-packages/google/genai/_transformers.py", line 849, in t_schema
    process_schema(schema, client)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
  File ".../genkit-intro/lib/python3.13/site-packages/google/genai/_transformers.py", line 775, in process_schema
    properties[name] = _recurse(sub_schema)
                       ~~~~~~~~^^^^^^^^^^^^
  File ".../genkit-intro/lib/python3.13/site-packages/google/genai/_transformers.py", line 747, in _recurse
    if (ref := sub_schema.pop('$ref', None)) is not None:
               ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'pop'