mocking overrides: default to concrete empty object when values are missing

[liamcervante]

When the values or outputs attribute was missing from the override_* blocks in the new mocking framework, we were setting the value to be cty.NilVal. This was then causing a crash later in the processing when Terraform was assuming the values in these attributes was an object.

From the perspective of the mocking framework a nil or empty value is the same, in both cases it should just generate the values required. This PR updates the config parsing so that it just sets the value to be an object always using cty.EmptyObjectValue if the attribute isn't specified. It also updates the Terraform graph where this is read so that it can safely handle a cty.NilVal anyway.

We also add in a quick bit of validation, that makes sure the config specifies the attributes as an object, if it is specified.

Fixes #34562

Target Release

1.7.1

Draft CHANGELOG entry

BUG FIXES

• terraform test: Fix crash when override_module block was missing the outputs attribute.

[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]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.