terraform: Diff.Empty should be true for nil Diff

terraform/diff.go

@@ -72,6 +72,10 @@ func (d *Diff) RootModule() *ModuleDiff {
 
 // Empty returns true if the diff has no changes.
 func (d *Diff) Empty() bool {
+	if d == nil {
+		return true
+	}
+
 	for _, m := range d.Modules {
 		if !m.Empty() {
 			return false

terraform/diff_test.go

@@ -7,7 +7,12 @@ import (
 )
 
 func TestDiffEmpty(t *testing.T) {
-	diff := new(Diff)
+	var diff *Diff
+	if !diff.Empty() {
+		t.Fatal("should be empty")
+	}
+
+	diff = new(Diff)
 	if !diff.Empty() {
 		t.Fatal("should be empty")
 	}