Dhtmlxgrid - combobox filtering

Hello,

I’ve got a problem with dhtmlxgrid and combobox filtering.
The default value “empty string” of the combobox filtering select all the row of the grid.
The problem is that I’ve got value “empty string” for this column.
Is there a way to change the default value like “*” for element?
The combolist is showing 2 empty slots which is a little weird to display. (as shown below)

image

Thank you for your feedback!

Paolo