Depends on how you are calculating the grand total now. Assuming you have an expression somewhere in query which is like:
SUM(SomeColumn) as GrandTotalYou would change that to
SUM(CASE WHEN YourColumnThatHasBlanks = '' THEN 0 ELSE SomeColumn END) as GrandTotalThis only a template for you to do what you need to do. Whether you check for empty strings ('') or NULL, or something else depends on your data type and data stored in blank rows for that column.