call-swarm: avoid dummy call to create a conference

Now a conference can start without any call (this avoid to attach
a call without peer to create a conference).

Path is now clearer for a call creation:
+ If we receive a call, we attach it to the conference
+ Else we do not create any call and just attach the host.

Next step is to be able to attach a host in audio only without weird
tricks and group addParticipant/bindParticipant

GitLab: #953
Change-Id: I13785a5525e041c37fb62c0c9f355e9371f1e4ad