Hi. Thanks for the project!

My OpenAPI spec has some fields as both nullable and required, but they are generating models with default values like this:

class Person(BaseModel):
    first_name: Annotated[str | None, Field(alias='firstName')] = None
    last_name: Annotated[str | None, Field(alias='lastName')] = None

Those default values make the fields optional, so it's not working as expected.

This fix also highlighted what looks to be a bug in an existing test that is checking for an incorrect result:

• JSON spec has two required fields:
  • https://github.com/koxudaxi/datamodel-code-generator/blob/main/tests/data/jsonschema/null_and_array.json
• Expected Pydantic V1 model looks good, both fields must be provided:
  • https://github.com/koxudaxi/datamodel-code-generator/blob/main/tests/data/expected/main/jsonschema/null_and_array.py
• Expected Pydantic V2 model looks wrong, one field has a default value so can be omitted:
  • https://github.com/koxudaxi/datamodel-code-generator/blob/main/tests/data/expected/main/jsonschema/null_and_array_v2.py