Fix for undefined DAEMON_ARGS warning fixes #1612

If DAEMON_ARGS isn't set in the environment file or the file doesn't
exist on systemd will output a warning see
systemd/systemd@f331434

This sets DAEMON_ARGS to an empty value if not set.

doc/example/service/linux/systemd/munin-node.service

@@ -6,7 +6,7 @@ Documentation=man:munin-node(8) http://guide.munin-monitoring.org/en/latest/refe
 EnvironmentFile=-/etc/default/munin-node
 Type=forking
 Restart=always
-ExecStart=/usr/sbin/munin-node $DAEMON_ARGS
+ExecStart=/usr/sbin/munin-node ${DAEMON_ARGS- }
 PIDFile=/run/munin/munin-node.pid
 
 [Install]