ISNULL

From AgileApps Support Wiki

The ISNULL function determines if a text field contains data. If the field has no data (value is null or empty), the function returns TRUE. If the field contains data, the function returns FALSE.

Syntax
ISNULL(field)
Return
TRUE or FALSE
Examples
ISNULL(last_name) = TRUE
IF(ISNULL(NULLDATE()), 'TRUE', 'FALSE')