handle null and unknown values in attr diffs

[jbardin]

The code adopted from block diffs was not set to handle null and unknown
values, as those are not allowed for blocks. While there is much room for
improvement in the formatting code, this PR takes care to only update the
new code paths for formatting so as to not disturb the rather fragile existing
block formatting code.

We also revert the change to formatting nested object types as single
attributes, because the attribute formatter cannot handle sensitive
values from the schema. This presents some awkward syntax for diffs for
now, but should suffice until the entire formatter can be refactored to
better handle these new nested types.

Fixes #29392

[alisdair]

This looks like a good compromise to me! I don't see any neat way to render empty collections on a single line without a big restructure, and the slightly less pretty diff doesn't feel like that big of a deal.

[github-actions]

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