config: DefaultsMap should return nil if nil

config/config.go

@@ -283,8 +283,11 @@ func (r *Resource) mergerMerge(m merger) merger {
 
 // DefaultsMap returns a map of default values for this variable.
 func (v *Variable) DefaultsMap() map[string]string {
-	n := fmt.Sprintf("var.%s", v.Name)
+	if v.Default == nil {
+		return nil
+	}
 
+	n := fmt.Sprintf("var.%s", v.Name)
 	switch v.Type() {
 	case VariableTypeString:
 		return map[string]string{n: v.Default.(string)}

config/config_test.go

@@ -102,6 +102,11 @@ func TestVariableDefaultsMap(t *testing.T) {
 		Default interface{}
 		Output  map[string]string
 	}{
+		{
+			nil,
+			nil,
+		},
+
 		{
 			"foo",
 			map[string]string{"var.foo": "foo"},