Helder consegui resolver, ficou assim
IF @object_type='17' AND @transaction_type in ('A', 'U') BEGIN
IF ((SELECT Top 1 U_Segmento FROM RDR1 WHERE DocEntry = @list_of_cols_val_tab_del) = '' Or
((Select Top 1 U_Segmento FROM RDR1 WHERE DocEntry = @list_of_cols_val_tab_del) is NULL))
BEGIN
Set @error = 1;
Set @error_message = 'Favor preencher o campo segmento na linha do produto/serviço!'
END
END
Abs
