The SalesOrderLine.ScheduledDate property is a Date type. The Date type is a value-type and therefore cannot be set to NULL. Normally when dealing with the Date type, in programming circles, DateTime.MinValue is used to denote NULL or "no date". Sadly Jiwa does some silly things with the saving of SalesOrderLine.ScheduledDate when it is set to DateTime.MinValue (Jiwa saves NULL to the database) and with the reading of SalesOrderLine.ScheduledDate (if null then use Today). This behaviour thwarts my attempts to use the DateTime.MinValue-as-NULL technique.
HOWEVER, in a similar vein to the aforementioned strategy, I have instead leveraged DateTime.MaxValue and jumped through a hoop or two to achieve what you want. Best to look at the plugin for an understanding. In a nutshell, we are using a special known date value, and when we come across said value during display, we set the cell.Value to NULL, which gives a blank cell. Note that reports, etc. must be made intelligent enough to know about the special known date value in SO_Lines.ScheduledDate (in this case DateTime.MaxValue) and treat it accordingly.