mctp-demux.service: Set a default binding to be overridden

This way the service at least won't fail out of the box.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ia1d33aa59bea53ea1338c70d9d7942b54de12f37
diff --git a/mctp-demux.service b/mctp-demux.service
index c04a46d..ad09b30 100644
--- a/mctp-demux.service
+++ b/mctp-demux.service
@@ -3,6 +3,7 @@
 
 [Service]
 Restart=always
+Environment=DEMUX_BINDING_OPTS=null
 EnvironmentFile=-/etc/default/mctp
 ExecStart=/usr/bin/mctp-demux-daemon $DEMUX_BINDING_OPTS
 SyslogIdentifier=mctp-demux