Fix checkin time zero json error

Signed-off-by: Asai Neko <sugar@sne.moe>

service/user/query.go

@@ -2,6 +2,7 @@ package user
 
 import (
 	"nixcn-cms/data"
+	"time"
 
 	"github.com/gin-gonic/gin"
 	"github.com/google/uuid"
@@ -30,7 +31,13 @@ func Query(c *gin.Context) {
 		return
 	}
 
+	var checkinTime *time.Time
+	if data.Checkin[eventId].(*time.Time).IsZero() {
+		checkinTime = nil
+	} else {
+		checkinTime = data.Checkin[eventId].(*time.Time)
+	}
 	c.JSON(200, gin.H{
-		"checkin_time": data.Checkin[eventId],
+		"checkin_time": checkinTime,
 	})
 }