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)

• [x] The commits have meaningful messages
• [x] 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).
• [x] The developer has manually tested the changes and verified that the changes work.
• [x] Code changes follow the kubeflow contribution guidelines.

[google-oss-prow[bot]]

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[pboyd]

I'm closing this because the fix is worse than the problem it's trying to solve.