fix(time): binding time with empty value (#4103)

* fix: binding time with empty value (#4098)

* refact: simplify null-to-zero filling logic

* test: add test for zeroUnixNanoTime

binding/form_mapping.go

@@ -397,6 +397,11 @@ func setTimeField(val string, structField reflect.StructField, value reflect.Val
 		timeFormat = time.RFC3339
 	}
 
+	if val == "" {
+		value.Set(reflect.ValueOf(time.Time{}))
+		return nil
+	}
+
 	switch tf := strings.ToLower(timeFormat); tf {
 	case "unix", "unixmilli", "unixmicro", "unixnano":
 		tv, err := strconv.ParseInt(val, 10, 64)
@@ -420,11 +425,6 @@ func setTimeField(val string, structField reflect.StructField, value reflect.Val
 		return nil
 	}
 
-	if val == "" {
-		value.Set(reflect.ValueOf(time.Time{}))
-		return nil
-	}
-
 	l := time.Local
 	if isUTC, _ := strconv.ParseBool(structField.Tag.Get("time_utc")); isUTC {
 		l = time.UTC