locking all records help please

This script works fine apart from fields which are 'empty'.

do as server
    for i in (select 'DOCUMENT REPOSITORY')[LOCKED = true]  do
        i.(LOCKED := false)
    end
end;
alert("All records unlocked")

In other words neither Yes or No.  The field was once 'not required' so a load of records are 'empty' rather than either Yes or No.  Can the above code be modified so that if neither Yes or No it will set Yes if Locking and No if Unlocking.

I have the opposite for vice/versa of course.