Filtering NULL Values in TDBAdvGrid

I use DBGrid with PageMode:=False.

Normal Sorting and Filtering works fine.
My DB Table is a TO_DO_LIST with a column FINISHDATE.
There are many tasks at table. If it's a new entry, FINISHDATE is NULL.
If the task is finished, FINISHDATE is filled with actual date.
If I show this table in a DBGrid,
at unfinished tasks the column "Finished on" is empty
at finished tasks the column "Finished on" holds a date.
Now I want t apply a filter "show only finished" or "show only unfinished".
So my question is: how to apply a filter on a column:
that shows only Rows with empty Cells
that shows only Rows with not empty Cells

Did you try to set a condition like :  >""

to see all cells that are not empty and
="" to see cells that are empty

Thank you, it works.

@ all other readers:
I've tried this before but I forgot to set
FilterActive := False;
before changing any filter, and
FilterActive := True;
after ALL filter changes are finished.