Ignore form field insert if blank

I have a form field that is shown/hidden depending on a radio selection before it.

If it is hidden then the value will be blank and the database will use the default value of 1.

If it is shown then it is validated to require a value of 2 to 10 which needs to be inserted.

If it is hidden and has a blank value how do get the database insert to ignore it?

Thanks in advance!