Import-csv and empty fields

I have an Excel file that I save as a csv to update users' offices, phone numbers, etc. Some of the fields in the CSV are blank. Blank fields will remove existing phone numbers and populating the blank fields with $null results in $null being set as the phone number.

import-csv "c:\admin\test\teamtest.csv"|foreach {set-qaduser -id $_.displayname -office $_.office -phonenumber $_.phonenumber}

How can I make import-csv bypass setting (or clearing) any value for a blank field?

I'm using Active Roles 1.4.

Thank you,
Robert