What do you mean, NULL is NULL (.NIL is .NIL)
and default to what ? NULL or .NIL or "something else", if "something else" the I suggest you populate the tablevalue with "something else" when tablevalue is written.

If I do this:

b = a~exec("select * from tbl1",.true,.oosqlite~oo_array_of_array)

then b will contain x number of arrays, where some of the values might by The NIL Object (.NIL) indicating that the value is NULL in the db.
Depending on the code one may have to manually test for .NIL to avoid syntax errors and other unwanted situations when manipulating the array values.
By doing for example:

a~defaultNULL = ""

one can be sure that no .NIL values exist in the arrays. Or one might use a~defaultNULL = "NULL" or a~defaultNULL = "Undecided" or whatever might be best in that situation.

I hope this helps.