vtysh: Ensure an empty string does not get printed for host/domain

vtysh: Ensure an empty string does not get printed for host/domain

vtysh show running-config is showing:
frr version 8.3.1_git
frr defaults traditional
hostname test
log file /etc/frr/frr.log informational
log timestamp precision 3
domainname
service integrated-vtysh-config

domainname should not be printed in this case at all. If the
host has no search/domainname configured, frr_reload.py
crashes on invalid config from `vtysh show running-config`

Basically the same change as commit a7141b8

Signed-off-by: Nico Berlee <nico.berlee@on2it.net>