HI David,
Try this :
If Isnumeric(ToNumber({RVS_CASE_MASTERS.UDF8})) Then
{RVS_CASE_MASTERS.DISCHDT} in {?beg_date} to {?end_date} and
ToNumber({RVS_CASE_MASTERS.UDF8}) >= 1
This will chek whether UDF8 is number or not, if it is number only it will execute this formula.
Also while making this formula, select 'Default Values for Null' from formula editor drow down