Tim's answer is correct at the update side, of course you could make the fields default to 0.
To get the results out through ISP, you may need to do something different in your select
myselstr = SELECT a,b, DECODE(numfield, NULL,'0', TO_TEXT(numfield)) from whatever;
sqlexec(myselstr);
If you need to do calculations from the select, just convert to text at the output