I've just been importing some CSV data, including one file column. Which all works beautifully, except that the import process stops and throws an error if that column contains any empty fields: it appears not to notice that it's a blank value and submits it to the processing code regardless.
I've solved this by hacking the models/CSVImport/Import.php script in the plugin folder, changing line #251:
if (!in_array($urlForFile, $urlsForFiles)) {
to:
if ((!in_array($urlForFile, $urlsForFiles)) && ($urlForFile != "")) {
...which seems to work fine.
Just thought I'd document the hack in case it's useful to anyone else.