Mark Finding properties related_fields, jira_creation and jira_change as nullable

[ccronca]

Description

Currently the OpenAPI spec lists the properties related_fields, jira_creation and jira_change as required in Findings objects. However, response may include these fields as null if they are not set.

This PR updates the specification to mark these properties as nullable. This is useful for generating clients from the openapi spec.

Test results

I don't believe there are tests for this.

Documentation

This is just a change to the openapi spec.

Checklist

This checklist is for your information.

• [ ] Make sure to rebase your PR against the very latest dev.
• [ ] Features/Changes should be submitted against the dev.
• [x] Bugfixes should be submitted against the bugfix branch.
• [x] Give a meaningful name to your PR, as it may end up being used in the release notes.
• [x] Your code is flake8 compliant.
• [x] Your code is python 3.11 compliant.
• [ ] If this is a new feature and not a bug fix, you've included the proper documentation in the docs at https://github.com/DefectDojo/django-DefectDojo/tree/dev/docs as part of this PR.
• [ ] Model changes must include the necessary migrations in the dojo/db_migrations folder.
• [ ] Add applicable tests to the unit tests.
• [ ] Add the proper label to categorize your PR.