have TestCheckResourceAttr accept missing counts

Missing containers were often erroneously kept in the state, but since
the addition of the new provider shims, they can often be correctly
eliminated. There are however many tests that check for a "0" count in
the flatmap state when there shouldn't be a key at all. This addition
looks for a container count key and "0" pair, and allows for the key to
be missing.

There may be some tests negatively effected by this which were
legitimately checking for empty containers, but those were also not
reliably detected, and there should be much fewer tests involved.

helper/resource/testing.go

@@ -971,7 +971,19 @@ func TestCheckModuleResourceAttr(mp []string, name string, key string, value str
 }
 
 func testCheckResourceAttr(is *terraform.InstanceState, name string, key string, value string) error {
+	// Empty containers may be elided from the state.
+	// If the intent here is to check for an empty container, allow the key to
+	// also be non-existent.
+	emptyCheck := false
+	if value == "0" && (strings.HasSuffix(key, ".#") || strings.HasSuffix(key, ".%")) {
+		emptyCheck = true
+	}
+
 	if v, ok := is.Attributes[key]; !ok || v != value {
+		if emptyCheck && !ok {
+			return nil
+		}
+
 		if !ok {
 			return fmt.Errorf("%s: Attribute '%s' not found", name, key)
 		}

helper/resource/testing_test.go

@@ -1113,3 +1113,34 @@ resource "test_instance" "foo" {}
 const testConfigStrProvider = `
 provider "test" {}
 `
+
+func TestCheckResourceAttr_empty(t *testing.T) {
+	s := terraform.NewState()
+	s.AddModuleState(&terraform.ModuleState{
+		Path: []string{"root"},
+		Resources: map[string]*terraform.ResourceState{
+			"test_resource": &terraform.ResourceState{
+				Primary: &terraform.InstanceState{
+					Attributes: map[string]string{
+						"empty_list.#": "0",
+						"empty_map.%":  "0",
+					},
+				},
+			},
+		},
+	})
+
+	for _, key := range []string{
+		"empty_list.#",
+		"empty_map.%",
+		"missing_list.#",
+		"missing_map.%",
+	} {
+		t.Run(key, func(t *testing.T) {
+			check := TestCheckResourceAttr("test_resource", key, "0")
+			if err := check(s); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}