by default this is not the widget responsability to turn empty string into None,
move this behaviour to the field.
/var/log/cubicweb/*.log {
weekly
missingok
rotate 10
compress
delaycompress
notifempty
create 640 root adm
sharedscripts
postrotate
if [ -x /usr/sbin/invoke-rc.d ]; then \
invoke-rc.d cubicweb reload > /dev/null; \
else \
/etc/init.d/cubicweb reload > /dev/null; \
fi; \
endscript
}