Found an article to explain the difference between nullable and optional in OpenAPI 3. But it's in Japanese.

I hate Katakana.

レスポンス -> Response
データ -> Data
プロパティ->Property
オプショナル->Optional

Just slap them into Google translator and listen how they sound like.

Then you will likely to ask yourself: Why?
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And just find out if you force Google translator to speak English in Japanese, you get the typical stereotype sounds of how Japanese people speak English.