Remove marking of entire object as unknown when child attribute is unknown

[austinvalle]

This PR:

• Removes the logic that marks an entire object as unknown when one of the child attributes is unknown
• Adds a final diag check before returning to avoid any weird object states (i.e. an object is known, but has a bunch of nil values because it had diags 😆 )

[austinvalle]

I did some testing with the petstore workflow example (where I originally ran into invalid plan errors due to a nested attribute being marked as unknown):

resource "petstore_pet" "clifford" {
  id   = 12345
  name = "Clifford the Big Red Dog"
  photo_urls = [
    "https://example.com/pet.jpg"
  ]
  category = {
    id = 1
   # name is a computed/optional field! 
  }
}

Now the terraform apply works 🚀 (the provider update code just sets unknown stuff to null)

 $ terraform apply -auto-approve
╷
│ Warning: Provider development overrides are in effect
│ 
│ The following provider development overrides are set in the CLI configuration:
│  - hashicorp.com/edu/petstore in /Users/austin.valle/go/bin
│ 
│ The behavior may therefore not match any released version of the provider and applying changes may cause the state to become incompatible with published releases.
╵
petstore_pet.clifford: Refreshing state... [name=Clifford the Big Red Dog]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # petstore_pet.clifford will be updated in-place
  ~ resource "petstore_pet" "clifford" {
      + category   = {
          + id   = 1
          + name = (known after apply)
        }
        id         = 12345
        name       = "Clifford the Big Red Dog"
      + status     = (known after apply)
      + tags       = (known after apply)
        # (1 unchanged attribute hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.
petstore_pet.clifford: Modifying... [name=Clifford the Big Red Dog]
petstore_pet.clifford: Modifications complete after 0s [name=Clifford the Big Red Dog]

Apply complete! Resources: 0 added, 1 changed, 0 destroyed.

[bflad]

This seems like better behavior for this functionality 👍

[github-actions]

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