[Feature] Support UnmarshalJSON of empty strings

[shuqingzai]

Test Cases

func TestDecimalUnmarshalJSON(t *testing.T) {  
    t.Run("number", func(t *testing.T) {  
       type number struct {  
          Name    string  `json:"name"`  
          Decimal Decimal `json:"decimal"`  
       }  
       jsonStr := `[{"name": "testFloat", "decimal": 1.1},{"name": "testInt", "decimal": 10},{"name": "testString", "decimal": "10.11"},{"name": "testEmpty",""},{"name": "testNone"}]`  
       var got []number  
       err := json.Unmarshal([]byte(jsonStr), &got)  
       if err != nil {  
          t.Errorf("UnmarshalJSON(%q) failed: %v", jsonStr, err)  
       }  
       if got[0].Decimal != MustParse("1.1") {  
          t.Errorf("UnmarshalJSON(%q) = %q, want %q", jsonStr, got[0].Decimal, MustParse("1.1"))  
       }  
       if got[1].Decimal != MustParse("10") {  
          t.Errorf("UnmarshalJSON(%q) = %q, want %q", jsonStr, got[1].Decimal, MustParse("10"))  
       }  
       t.Logf("got: %+v", got)  
    })
}

When decimal is an empty string(testEmpty), UnmarshalJSON cannot be performed. Is it possible to support this? The default value is zero? Or can global configuration be added to support this option?

Background Information

When we connect to some third-party libraries, we often encounter empty string values ​​and cannot support UnmarshalJSON

[FlameMida]

same problem