Correct bad DX/UX of configuration

[AbstractUmbra]

Description

With the existing setup, our config allows for child keys to be optional, meaning a parent key is required but can be empty?

[REDIS]
# no child keys

[OTHER_KEY]
some_key = some_value

I doubt this can cause parsing issues but it doesn't look particularly good.
This PR corrects this, meaning the parent key is now the optional one, not the child keys. If the parent key is present then the child keys are required.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
  • I have updated the changelog with a quick recap of my changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)
• I have read and agree to the Developer Certificate of Origin for this contribution