Fix Pydantic Creator Model: Don't mark fields optional if default value is not null

[fullonic]

TODO: Add a test before open PR to the ORM repo

Description

Motivation and Context

How Has This Been Tested?

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Copilot]

Pull Request Overview

This PR adjusts the logic for marking fields as Optional in the Pydantic creator model so that only fields with a None default (rather than any non-None default) become optional.

• Inverts the default check: now Optional is applied only when field.default is None.
• Retains existing behavior for primary keys and nullable fields.
• No other files are modified.

Comments suppressed due to low confidence (3)

tortoise/contrib/pydantic/creator.py:528

• Add a unit test to verify that fields with a non-None default no longer become Optional, and that fields with None defaults still do.

            field_name in self._optional or field.default is None or field.null

tortoise/contrib/pydantic/creator.py:528

• [nitpick] Consider handling default_factory (if supported by the field) similarly to explicit defaults, so fields with a factory-provided default aren’t incorrectly marked optional.

            field_name in self._optional or field.default is None or field.null

tortoise/contrib/pydantic/creator.py:524

• [nitpick] Update or add a brief docstring explaining the combined logic for default, nullable, and _optional flags when determining Optional typing for clarity.

        ptype = python_type