[views] csvexport accept an empty rset (closes #4236928) When you tried to apply the 'csvexport' view on an empty rset, the view couldn't be selected and you got a HTTP 500 error. Also add two new test cases.

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0