Backport of Return error when the template's single interpolation results in null value into v1.11

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Backport

This PR is auto-generated from #36658 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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Fixes #36652

When a string interpolation contains only one expression sequence, HCL optimizes by wrapping and directly delegating evaluation to that expression. The templatestring function tries to refine the expression value as non-null, and if null, this refinement leads to a panic. For compatibility reasons, HCL has to continue to support this unwrapping logic (See attempted PR), thus we will simply return an error in templatestring when the single evaluated expression is null

Target Release

1.12.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

• 10ddb3b

• 031fb73

• 774192b

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