DateTime Should update if the data is passed else update it with original value which is already saved. This update does not works

```
DECLARE @FolderStatusDate DATETIME = NULL

SET @FolderStatusDate = '2012-07-04 14:09:04.043'

UPDATE CM.PfmFolder     
  SET   
      FolderStatusDate = ISNULL(@FolderStatusDate, FolderStatusDate)
      WHERE Id = @Id  

```