Unchanged NestedSets are not returned by DiffFieldReader.

[gotascii]

Given a schema such as:

schema := map[string]*Schema{
  "list_of_sets_1": &Schema{
    Type: TypeList,
    Elem: &Resource{
      Schema: map[string]*Schema{
        "nested_set": &Schema{
          Type: TypeSet,
          Elem: &Resource{
            Schema: map[string]*Schema{
              "val": &Schema{
                Type: TypeInt,
              },
            },
          },
          Set: func(a interface{}) int {
            m := a.(map[string]interface{})
            return m["val"].(int)
          },
        },
      },
    },
  },
  "list_of_sets_2": &Schema{
    Type: TypeList,
    Elem: &Resource{
      Schema: map[string]*Schema{
        "nested_set": &Schema{
          Type: TypeSet,
          Elem: &Resource{
            Schema: map[string]*Schema{
              "val": &Schema{
                Type: TypeInt,
              },
            },
          },
          Set: func(a interface{}) int {
            m := a.(map[string]interface{})
            return m["val"].(int)
          },
        },
      },
    },
  },
}

And a configuration such as:

resource "provider_name_resource_name" "test" {
  list_of_sets_1 {
    nested_set {
      val = 1
    }
  }
  list_of_sets_2 {
    nested_set {
      val = 1
    }
  }
}

Updating the val of the nested_set inlist_of_sets_1 from 1 to 2:

resource "provider_name_resource_name" "test" {
  list_of_sets_1 {
    nested_set {
      val = 2
    }
  }
  list_of_sets_2 {
    nested_set {
      val = 1
    }
  }
}

Results in resourceData.Get("list_of_sets_2") returning an empty *schema.Set.

This PR provides a unit test along with a fix. The fix is to check if the state has a value for the field after determining that the value does not exist in the diff. This matches the behavior of the primitive and other field types.

I ran all of the other core tests and they continue to pass, so I think the fix is not inadvertently breaking anything. Would love feedback on how to get this PR to a state where it is mergable! Thanks!

[phinze]

This looks great - thanks @gotascii!

[ghost]

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