Restoring a null value to a data table

Have you ever inadvertently deleted the contents of a data table cell that contained a null value and wondered how to restore it? The support log suggests that a lot of us forget how to do that.
First a little background: There are several AscentialTest data components where a null character might be stored. You will find them in the cells of data tables, test plan data objects and run arguments from the step or test editors. The null character is represented with a special character that looks like a small box.

Inputting a null value into a cell is easy if you know the key sequence. Just type the <Alt> key with the ‘n’ character (n for null).