Revert "Code field for ReaderEvent: Default empty string."

This reverts commit 0c194224.

MoyaAdmin/MoyaAdminUI/MoyaAdminLib/ReaderEvent.cs

@@ -14,6 +14,6 @@ namespace MoyaAdminLib
         public Eventuser eventuser;
         public int? printedCardId = 0;
         public string printedCardState = "";
-        public string code = "";
+        public string code;
     }
 }