CSV files & empty strings

I am new to Postgre, having formerly used Filemaker Pro. I need to import
CSV files into Postgre which contain some empty string "" values in several
date-type fields. I am able to import the CSV files into Postgre with the
COPY FROM command if the datefields are converted to text fields in
Postgre. However, I need some method of "converting" the empty string ""
values into NULL values after import, so that I can change the date fields
back to date-type. Any suggestions on how this could be accomplished?

Thanks,

-Nate