In datasheet view for a table you have a choice of no-value, or null, or empty for a field acting as a foreign key in a one-many relationship with data-integrity enforced. This is as expected. However, I am unable to replicate this behaviour in a datapage with the lookup table. If you try then you get a data integrity error

See: http://forums.caspio.com/index.php/topic/5730-how-to-enter-empty-null-into-table-field-with-relationship-from-data-page/