Backport of backends: Fix crash when interrupting during interactive prompt for values' into v1.11

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Backport

This PR is auto-generated from #36448 to be assessed for backporting due to the inclusion of the label 1.11-backport.

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

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This PR fixes a crash that affects all remote state backends that implement the backendbase.Base. When interrupting (Ctrl+c) during Terraform's interactive value prompt, we convert the resulting unknown value to a null value.
Our generic PrepareConfig wasn't prepared to handle nulls and crashed when applying the sdk-like defaults.

Fixes #36420

Target Release

1.11.x

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

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Overview of commits

• e730103

• 1085494

• de0768c

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