mgcp/sdp: Session name must not be empty pick an empty one

The session name must be present in a SDP file. The RFC proposes
to use a space for it but the other equipment is using the dash
so I have picked that as well.

RFC 4566:
The "s=" field is the textual session name.  There MUST be one and
only one "s=" field per session description.  The "s=" field MUST NOT
be empty and SHOULD contain ISO 10646 characters (but see also the
"a=charset" attribute).  If a session has no meaningful name, the
alue "s= " SHOULD be used (i.e., a single space as the session
name).

Fixes: RT#2196