SELE ...fields... ;
  FROM customers LEFT [OUTER] JOIN invoices;
    ON customers.customer_id = invoices.customer_id;
  WHERE invoices.order_dt BETWEEN.... ;
     OR ISNULL(Invoices.Order_dt) ...

Your query will not fix the problem because the record with NULL will not be included in the result. Adding the above OR condition should work although it would need to be tested to see if the non-invoices show as NULL or empty.