[Backend] 修復 SaveDeviceKind 上傳圖片問題

Backend/Controllers/DeviceManageController.cs

@@ -1281,7 +1281,9 @@ namespace Backend.Controllers
                     {
                         //刪除原本的檔案
                         FolderFunction folderFunction = new FolderFunction();
+                        if (deviceKind.Device_image != null) { 
                             folderFunction.DeleteFile(Path.Combine(deviceKindFileSaveAsPath, deviceKind.Device_image));
+                        }
 
                         var split = post.Device_image_file.FileName.Split(".");
                         var fileName = image_guid + "." + split[split.Length - 1];