fix: Empty string is not consistent #311 (#312)

Author: a8ss

type_carbon.go

@@ -55,6 +55,7 @@ func (c Carbon) MarshalJSON() ([]byte, error) {
 func (c *Carbon) UnmarshalJSON(src []byte) error {
 	v := string(bytes.Trim(src, `"`))
 	if v == "" || v == "null" {
+		c.isEmpty = true
 		return nil
 	}
 	*c = *ParseByLayout(v, DefaultLayout)

type_unit_test.go

@@ -149,6 +149,8 @@ func (s *CarbonTypeSuite) TestCarbonType_UnmarshalJSON() {
 
 		s.Empty(model.Carbon1.String())
 		s.Empty(model.Carbon2.String())
+		s.False(model.Carbon1.IsValid())
+		s.False(model.Carbon2.IsValid())
 	})
 
 	s.Run("null value", func() {