Uploaded Static List With Accidental Empty Fields

Earlier this week I uploaded a large list. Aside from email address, columns in the list included Revenue and Number of Employee fields. This was an effort to append our database with updated demographic information. What I did not realize is that a good portion of the contacts in the list had nothing in those fields. My question is as follows:

If I upload a list and one of the fields I upload is empty (not "NULL", but completely blank), will that overwrite data that may have already been in Marketo? If a contact did have something in the "Annual Revenue" field, and in the file I uploaded, that same contact had an empty field, would that empty value overwrite the pre-existing data? My hope is that data will only be overwritten in a list upload if the field said "NULL".

Thanks!

Steve