[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

if [ -x "/etc/init.d/cubicweb-ctl" ]; then
    invoke-rc.d cubicweb-ctl restart || true
fi
 
 
#DEBHELPER#
 
exit 0