swarm-call: avoid multiple call creation when receiving concurrent calls

A Jami client can receive several calls in parallel, trying to create/
initiate a rendezvous in a swarm at the same moment. Because callId
will be different for each, this will result in several rendezvous
with different peers.
Instead of this, all incoming calls can pass an invalid Id and
if no id specified, the host can create a first conference, or
just join incoming requests (that aren't a join to a specific id)
in one and unique call.

Change-Id: I4e648bec4205e7dcd8860c6140568afcba6f92ed