You can use NULL values in List & Label by passing a special string to the APIs. This means that this field has no current value, e.g. a delivery date for a shipment that has not yet occurred. Most database drivers may return a field content of NULL, which you need to pass on to List & Label as "(NULL)", although that string can be altered if needed. Basically the List & Label components handle database NULL values automatically.
List & Label handles NULL-values according to the SQL-92 specification where possible. An important effect of that is, that functions and operators, which get NULL-values as parameter or operator generally also return NULL as the result. An example is the following Designer formula:
Title+" "+Firstname+" "+Lastname
If Title is filled with NULL, the result of the formula is also NULL according to the standard. To change this behaviour please refer to the option LL_OPTION_NULL_IS_NONDESTRUCTIVE.