fix(openapi): Mark Region.main_generation as nullable (#1486)

* fix(openapi): Mark Region.main_generation as nullable

The Hisui region (and potentially others) have no main_generation,
causing the API to return null for this field. The OpenAPI spec was
incorrectly marking it as required and non-nullable.

Fixes #1397

* fix: Allow null for Region.main_generation in serializer

Some regions (e.g. Hisui) have no main_generation, so the API returns
null for this field. Adding allow_null=True to the serializer field
so the generated OpenAPI spec correctly marks it as nullable.

Fixes #1397

* style: Run black and regenerate openapi.yml

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Co-authored-by: santichausis <santichausis@users.noreply.github.com>

Author: santichausis