fix: 规范化返回当 errors 为 null 时，现在 message 返回 “”，之前是 "null"

Admin.NET/Admin.NET.Core/Util/AdminResultProvider.cs

@@ -91,7 +91,7 @@ public class AdminResultProvider : IUnifyResultProvider
         return new AdminResult<object>
         {
             Code = statusCode,
-            Message = errors is string str ? str : JSON.Serialize(errors),
+            Message = errors is null or string ? (errors + "") : JSON.Serialize(errors),
             Result = data,
             Type = succeeded ? "success" : "error",
             Extras = UnifyContext.Take(),