Backport of plan renderer: fix crash when updating a null attribute to unknown into v1.9

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Backport

This PR is auto-generated from #35709 to be assessed for backporting due to the inclusion of the label 1.9-backport.

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

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This PR fixes a crash in the plan renderer that occurs when a plan marks an attribute that was null as becoming unknown.

The renderer had assumed that all computed attributes would have a value if the resource was being updated, so was only checking for plans.Create actions before deciding whether to render the "before" part of an unknown change. Now, we explicitly check whether the before part has a renderer at all when deciding whether to render the before part rather than simply relying on a logical consistency based on the false assumption it would only ever be create actions doing this.

There's also a flyby fix of the block renderers which can now successfully render blocks becoming unknown as update actions instead of create since that does not crash anymore.

Note, we could render this as something like ~ attr = null -> (known after apply), instead of just ~ attr = (known after apply), but I don't think we do that elsewhere (like when an optional attribute is set during an update operation) so I didn't implement it here but it would be trivial to do.

Fixes #35630

Target Release

v1.9.7

Draft CHANGELOG entry

BUG FIXES

• Plan renderer: Fix crash that occurs when updating a null attribute to unknown

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Overview of commits

• cf16330

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Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.