HELP - Big Problem with Form

I used data assist to create admin pages for my ecart store where an admin can add a new product.

My digital goods and physical goods are separated in the download centre based on whether the product location field is NULL (NON DIGITAL) or a server file name (DIGITAL).

To make it easy, I use the same form to allow users to add new physical and digital products.
My form comprises of all the needed fields, name, price stock etc..
And at the bottom is a file field which is used to upload a digital good. If the new product is not digital, they do not use it.

However my issue arises when the new product is not digital.

Even when the file field is not used it forces a blank value in the database and dose not allow a NULL field.

I have already set the database table filed default value to Null, but Digital file pro will not allow the database to be NULL it just forces a blank value in this field.

How can I get around this without having to create two separate forms? One for physical and another for digital.

Thank you