Use the IsNull property to discover if the value of the parameter is NULL, indicating the value of a blank field. NULL values can arise in the following ways:
• Assigning the value of another, NULL, parameter.
• Assigning the value of a blank TField object using the AssignFieldValue method.
• Calling the Clear method.
NULL parameters are not the same as unbound parameters. Unbound parameters have not had a value assigned. NULL parameters have a NULL value. NULL parameters may be bound or unbound.