Fix CSV exporter not using the user supplied NULL string.
Fix CSV exporter not using the user supplied NULL string.

CoolSwitcher/sequelpro,CoolSwitcher/sequelpro,leichunxin/sequelpro,eric-seekas/sequelpro,abhibeckert/sequelpro,chenbk85/sequelpro,abhibeckert/sequelpro,leichunxin/sequelpro,treejames/sequelpro,kaneshin/sequelpro,Jellyfrog/sequelpro,abhibeckert/sequelpro,kaneshin/sequelpro,chenbk85/sequelpro,leichunxin/sequelpro,treejames/sequelpro,Jellyfrog/sequelpro,CoolSwitcher/sequelpro,dnl-jst/sequelpro,chenbk85/sequelpro,abhibeckert/sequelpro,josegonzalez/sequelpro,CoolSwitcher/sequelpro,Jellyfrog/sequelpro,treejames/sequelpro,dnl-jst/sequelpro,eric-seekas/sequelpro,dnl-jst/sequelpro,dnl-jst/sequelpro,chenbk85/sequelpro,kaneshin/sequelpro,kaneshin/sequelpro,Jellyfrog/sequelpro,leichunxin/sequelpro,josegonzalez/sequelpro,josegonzalez/sequelpro,treejames/sequelpro,eric-seekas/sequelpro,josegonzalez/sequelpro,eric-seekas/sequelpro