Tried "" or '' but still no luck! Tested it on shapefile and FGDB feature class as well.
As for the field calculator expression, it looks empty but actually it is not. It always does not show for no reason.
The problem comes from the None or Null format in the date field because if I set any date instead, it does work. Also, it works in former ArcToolbox. Is it related to Python 3 ? There must be a solution...