jvb: fix specifying an empty list of STUN servers

Fixes: https://github.com/jitsi/docker-jitsi-meet/issues/43

jvb/rootfs/defaults/sip-communicator.properties

@@ -2,7 +2,9 @@ org.jitsi.videobridge.AUTHORIZED_SOURCE_REGEXP={{ .Env.JICOFO_AUTH_USER }}@{{ .E
 org.jitsi.videobridge.SINGLE_PORT_HARVESTER_PORT={{ .Env.JVB_PORT }}
 org.jitsi.videobridge.DISABLE_TCP_HARVESTER={{ .Env.JVB_TCP_HARVESTER_DISABLED }}
 org.jitsi.videobridge.TCP_HARVESTER_PORT={{ .Env.JVB_TCP_PORT }}
+{{ if .Env.JVB_STUN_SERVERS }}
 org.ice4j.ice.harvest.STUN_MAPPING_HARVESTER_ADDRESSES={{ .Env.JVB_STUN_SERVERS }}
+{{ end }}
 
 org.jitsi.videobridge.xmpp.user.shard.HOSTNAME={{ .Env.XMPP_SERVER }}
 org.jitsi.videobridge.xmpp.user.shard.DOMAIN={{ .Env.XMPP_AUTH_DOMAIN }}