backend/local: don't panic when an instance has only a deposed object

[apparentlymart]

This unusual situation isn't supposed to arise in normal use, but it can come up in practice in some edge-case scenarios where Terraform fails in a severe way during a create_before_destroy.

Some earlier versions of Terraform also had bugs in their handling of deposed objects, so this may also arise if upgrading from one of those older versions with some leftover deposed objects in the state.

This fixes #21396.

---

Some additional context:

In working on this I noticed that in principle the specific situation I covered in the test here could be simplified to just a normal create_before_destroy in the UI, because that is effectively what's going to happen anyway.

However, the UI handling of deposed instances is generalized to deal with a bunch of other similar edge-cases around deposed instances. The main one it's dealing with is the situation where the "destroy" part of a create_before_destroy fails and thus the state ends up containing both a current instance and a deposed instance at the same time. If the user then updates the config again and triggers another create_before_destroy then the destroy can fail again and accumulate two deposed instances, like this:

Terraform will perform the following actions:

  # test_instance.foo will be replaced
+/- resource "test_instance" "foo" {
        ami = "bar" -> baz

        network_interface {
            description  = "Main network interface"
            device_index = 0
        }
    }

  # test_instance.foo (deposed object ABCDABCD) will be destroyed
  - resource "test_instance" "foo" {
      - ami = "bar" -> null

      - network_interface {
          - description  = "Main network interface" -> null
          - device_index = 0 -> null
        }
    }

  # test_instance.foo (deposed object DEFADEFA) will be destroyed
  - resource "test_instance" "foo" {
      - ami = "foo" -> null

      - network_interface {
          - description  = "Main network interface" -> null
          - device_index = 0 -> null
        }
    }

The above situation is also rare, but it's less rare (easier to run into) than the one this fix is aimed at, and so that's why the UI treatment of deposed objects is designed around this latter case (where combining into a single action isn't possible) rather than the "deposed only" situation (where it could potentially be).

[ghost]

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