transform: test flatten mangler with nil subfields

I suspect that we're not handling nested pointer-types correctly in the
FlattenMangler. This adds a test that fails because unmangle's creating
empty intermediate structs.

transform/flatten_mangler_test.go

@@ -207,6 +207,65 @@ func TestFlattenMangler(t *testing.T) {
 				assert.Equal(t, st, i)
 			},
 		},
+		{
+			name:       "nil nested struct",
+			testStruct: b,
+			modify: func(t testing.TB, val reflect.Value) {
+
+				expectedDialsTags := []string{
+					"config_field_Name",
+					"config_field_Foobar_Location",
+					"config_field_Foobar_Coordinates",
+					"config_field_Foobar_some_time",
+					"config_field_AnotherField",
+				}
+
+				expectedFieldTags := []string{
+					"ConfigField,Name",
+					"ConfigField,Foobar,Location",
+					"ConfigField,Foobar,Coordinates",
+					"ConfigField,Foobar,SomeTime",
+					"ConfigField,AnotherField",
+				}
+
+				for i := 0; i < val.Type().NumField(); i++ {
+					assert.EqualValues(t, expectedDialsTags[i], val.Type().Field(i).Tag.Get(common.DialsTagName))
+					assert.EqualValues(t, expectedFieldTags[i], val.Type().Field(i).Tag.Get(dialsFieldPathTag))
+					if val.Type().Kind() != reflect.Pointer {
+						t.Errorf("field %d has kind %s, not %s", i, val.Type().Kind(), reflect.Pointer)
+					}
+				}
+
+				s1 := "test"
+				i2 := 42
+
+				val.Field(0).Set(reflect.ValueOf(&s1))
+				val.Field(1).Set(reflect.Zero(reflect.TypeOf((*string)(nil))))
+				val.Field(2).Set(reflect.Zero(reflect.TypeOf((*int)(nil))))
+				val.Field(3).Set(reflect.Zero(reflect.TypeOf((*time.Duration)(nil))))
+				val.Field(4).Set(reflect.ValueOf(&i2))
+			},
+			assertion: func(t testing.TB, i interface{}) {
+				// all the fields are pointerified because of call to Pointerify
+				s1 := "test"
+				i2 := 42
+				b := struct {
+					Name   *string `dials:"Name"`
+					Foobar *struct {
+						Location    *string `dials:"Location"`
+						Coordinates *int    `dials:"Coordinates"`
+						SomeTime    *time.Duration
+					} `dials:"Foobar"`
+					AnotherField *int `dials:"AnotherField"`
+				}{
+					Name:         &s1,
+					Foobar:       nil,
+					AnotherField: &i2,
+				}
+
+				assert.EqualValues(t, &b, i)
+			},
+		},
 		{
 			name:       "multilevel nested struct",
 			testStruct: b,