fix: 保守法设置 isIAPUser 属性

Signed-off-by: huyufei <yufei.hu@castbox.fm>

Runtime/GuruIAP/Runtime/Code/IAPServiceBase.cs

@@ -490,7 +490,7 @@ namespace Guru
                         isIAPUser = true;
                     }
                 }
-                SetIsIAPUser(isIAPUser);
+                if(isIAPUser) SetIsIAPUser(true);
             }
             
             OnRestored?.Invoke(success, msg);