Filter on empty value

we are using dhtmlxGrid to show some data that includes a date field.
The date is set when some processing happend. Our customer now wants to filter on
the “empty” date field, so he can see all entries that were not processed so far.
Simple sorting was not an option and placeholders are not the best solution.
So the question:
Is is possible to filter for empty values?

Unfortunately such feature is not available.