Backport of handle null and unknown values in attr diffs into v1.0

[ghost]

Backport

This PR is auto-generated from #29410 to be assessed for backporting due to the inclusion of the label 1.0-backport.

The below text is copied from the body of the original PR.

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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

[jbardin]

requires manual backport

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