feat: make catalog model fields nullable in OpenAPI spec

[pboyd]

Description

Add nullable: true to catalog model fields and make them required.

This allows the API output for a model to more closely match the catalog
file.

How Has This Been Tested?

Unit and E2E tests

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages
• Automated tests are provided as part of the PR for major new functionalities; testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please ask for approval from pboyd. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment