command/workspace_delete: Allow deleting a workspace with empty husks

[apparentlymart]

Previously we would reject attempts to delete a workspace if its state contained any resources at all, even if none of the resources had any resource instance objects associated with it.

Nowadays there isn't any situation where the normal Terraform workflow will leave behind resource husks, and so this isn't as problematic as it might've been in the v0.12 era (as discussed in #23269), but nonetheless what we actually care about for this check is whether there might be any remote objects that this state is tracking, and for that it's more precise to look for non-nil resource instance objects, rather than whole resources.

This also includes some adjustments to our error messaging to give more information about the problem and to use terminology more consistent with how we currently talk about this situation in our documentation and elsewhere in the UI.

We were also using the old State.HasResources method as part of some of our tests. I considered preserving it to avoid changing the behavior of those tests, but the new check seemed close enough to the intent of those tests that it wasn't worth maintaining this method that wouldn't be used in any main code anymore. I've therefore updated those tests to use
the new State.HasResourceInstanceObjects method instead.

This closes #23269.

[alisdair]

Looks good, with one question inline about data sources.

[apparentlymart]

With the new methods now only considering managed resources, State.HasManagedResourceInstanceObjects was no longer a suitable guard to fast-exit the StateTransformer, but that's not super important anyway since iterating over the contents of an empty state object produces the same effect anyway. I changed it only to exit early if the state is nil, which happens occasionally in our unit tests even though it's not really expected in main code (we prepare an empty state before beginning an initial plan).

Otherwise, this is as in my original changeset except that it ignores the presence of any non-managed resources, as suggested in comments above.

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