My form has the following fields: ID, DueDate, DatePaid,HowPaid,Check#,
AmtPaid, FeePaid, Gift, PastDue, Notes.

A DatePaid entry can be made for a Gift or Fee leaving the duedate field
empty. I would like to sort by the due date with the empty fields sorted by
datepaid. Right now the empty fields are on top of the sorted list. I do
not want to sort by datepaid when duedate field is not empty.