try fixing empty columns in CSV output

booking.variety_admin.inc

@@ -599,7 +599,7 @@ function booking_varietysessions_csv_report($timeslot_id) {
   for ($i = 0; $i < max($maximums); $i++) {
     $output = array();
     foreach ($header_array as $column) {
-      $field = isset($data[$column][$i]) ? $data[$column][$i] : '';
+      $field = isset($data[$column][$i]) ? $data[$column][$i] : ' ';
 
       //enclose $field if necessary
       if ( $encloseAll || preg_match( "/(?:${delimiter_esc}|${enclosure_esc}|\s)/", $field ) ) {