增加对包含null的json字符串的解码支持

encoding.go

@@ -33,6 +33,9 @@ func (c *Carbon) UnmarshalJSON(b []byte) error {
 	if c.Error != nil {
 		return c.Error
 	}
+	if string(b) == "null" || len(b) == 0 {
+		return nil
+	}
 	key, value, tz := c.parseTag()
 	data := fmt.Sprintf("%s", bytes.Trim(b, `"`))
 	if key == "layout" {

encoding_unit_test.go

@@ -3,9 +3,8 @@ package carbon
 import (
 	"encoding/json"
 	"fmt"
-	"testing"
-
 	"github.com/stretchr/testify/assert"
+	"testing"
 )
 
 type Person struct {
@@ -284,3 +283,26 @@ func TestError_Json(t *testing.T) {
 	fmt.Println("unmarshal error:", unmarshalErr.Error())
 	assert.NotNil(t, unmarshalErr)
 }
+
+// https://github.com/golang-module/carbon/issues/225
+func TestCarbon_Issue225(t *testing.T) {
+	str := `{
+		"birthday1":"",
+		"birthday2":null
+	}`
+
+	type Person struct {
+		Birthday1 Carbon `json:"birthday1"`
+		Birthday2 Carbon `json:"birthday2"`
+	}
+
+	var person Person
+	unmarshalErr := json.Unmarshal([]byte(str), &person)
+	fmt.Println(unmarshalErr) // nil
+
+	assert.Nil(t, unmarshalErr)
+
+	assert.Equal(t, "", person.Birthday1.String())
+	assert.Equal(t, "", person.Birthday2.String())
+	fmt.Println(person.Birthday1.String()) // empty string
+}