[rset] Fix regression with rset copying (closes #3344410)
Don't override passed-in descriptions in ResultSet.__init__, even if
they're empty.
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0