This is a long term issue: How to handle "Nulls". My personal preference is I don't allow nulls in my tables or database, either I will not write the empty column or I will give it a default value. This eliminates the problems when you allow the user to enter a value, if the entered field is text field datatype, users will enter a space, a tab key (to go to the next field), etc. If the field is a numeric type field, users may skip or space the numeric field. Numeric fields I would either place a "0" or some number "99999" to indicate an empty cell. In addition to default values, it is a good practice to use "validation" on fields you allow to be NULLs.
Hope this helps.
Disclaimer: The above comments are solely the opinion of one person and not to be construed as a directive or an incentive to commit fraudulent acts.