You can search for null and not null values by entering the following values into a search field on the Find tab:
- To search for a null value enter: ~null~
- To search for not null values enter: != ~null~
If your keyboard does not have the tilde '~' character you may type enter this character by typing 126 on your numeric keypad while pressing the 'Alt' key.
Maybe this sound obvious to you but it was not easy to discover...