Fix crash when generating config for a resource with complex sensitive attributes

[liamcervante]

This PR fixes a crash that occurs when generating configuration for a resource with "complex" sensitive attributes. Complex in this case means a list, set, map or object. Basically anything not a primitive type.

This bug was exposed by an earlier fix in which we started properly marking sensitive attributes within the plan, instead of waiting for the plan renderer to handle these directly (#34567). Now that sensitive marks will appear on values returned by the provider the config generation logic crashes as it is not checking for sensitive marks in the correct places.

The linked stack trace indicates this is happening during the omitUnknowns function. The fix in this case is to simply remove that function. As of #34525, it is impossible for us to encounter unknowns at this point anyway so the validation is just needless extra work.

There is also a problem with the check for "empty strings as null strings" required due to the legacy SDK. We need to unmark the string value before reading it. We don't need to do anything except restore the marks after the value has been changed into a null.

Fixes #34992

Target Release

1.8.1

Draft CHANGELOG entry

BUG FIXES

• Fix crash when generating configuration for resources with complex sensitive attributes.

[DanielMSchmidt]

Removing unnecessary code and fixing a panic sounds like a double treat 👍

[github-actions]

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

[github-actions]

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