Dhtmlxgrid null value behaviour explanation?

I load data in my grid from a .NET connector.
In the database, there is a date field that is non-mandatory ; so it contains NULL or a valid date.
When loaded in the grid, the unfilled dates (ie NULL) appears correctly (empty).
But when updating or inserting data, it is an empty string (which is not a NULL value) that is sent to the database . As a result, the database complains correctly :
cannot convert ‘’ to a timestamp.

Is there a way to tell the grid to handle NULL values correctly, taht is not confusing it with an empty string ?
PS : of course, everything runs fine when a date is specified, but I cannot put a date for all the records.