Fix framework setting nested blocks within nested lists to nil

[liamcervante]

Fixes issue raised in #479 (comment)

Apologies, not sure of the process for raising PRs but this seemed very simple so I thought I'd give it a go. Feel free to close this if this needs to be tracked in a different way.

[bflad]

Hi @liamcervante 👋 Thanks for submitting this fix. I think this is an appropriate code change that was missed in #468 so thank you for catching it.

Would you like to finish up this pull request or leave it for us? In order to get this in, let's:

• Ensure there is a changelog entry file created, e.g. .changelog/480.txt with contents such as:

```release-note:bug
internal/fwserver: Ensured nested block plan modifiers correctly set their request `AttributeConfig`, `AttributePlan`, and `AttributeState` values
```

• Ensure there is a covering unit test in block_plan_modification_test.go since it was already missing before. This may be non-trivial/verbose to implement, so don't feel like you should spend too much time here. The block-set-nested-usestateforunknown test case might be a good starting point to copy-paste-modify. 👍

Thanks again.

[liamcervante]

I will work on this today!

[liamcervante]

Ready for review!

[bflad]

Thanks so much, @liamcervante 🚀

[github-actions]

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