update rich text to allow null values

[bmigirl]

I want to merge this change because...

Screenshots

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. The changes are tested.
4. The code is documented (docstrings, project documentation).
5. Changes are mentioned in the changelog.

Test Environment Config

API_URI=https://master.staging.saleor.cloud/graphql/

[patrys]

Task linked: SALEOR-2270 Editor.js should handle null values in content fields

[jwm0]

Return statements should come after all hook declarations as this guarantees their call order during render phase:

https://reactjs.org/docs/hooks-rules.html#only-call-hooks-at-the-top-level