fix: correctly handle nullable: false

[darrenyong]

🚥 Resolves RM-11473

🧰 Changes

Currently, if a schema has nullable: false, (idk why since it defaults to false? maybe some tooling explicitly defines it as such) we aren't properly handling it as we're only checking for the existence of a nullable property.

I added a condition to only add the null type if nullable: true. If it's false, we don't want to do anything with it, but we still want to delete it so it doesn't show up in the schema. Added a test too.

🧬 QA & Testing

Provide as much information as you can on how to test what you've done.

[erunion]

lmk when you want to publish this and i can handle it for ya