[bfss] Fix update of BFSS attribute to None (close #1875289) The current code assume attr value is alway a valide Binary value. But when the attr is optional, the value might be None. The change handle this case.

#! /bin/sh -e
 
case "$1" in
    remove)
	if [ -x "/etc/init.d/cubicweb-ctl" ]; then
	    invoke-rc.d cubicweb-ctl stop || true
	fi
    ;;
esac
 
#DEBHELPER#
 
exit 0