Handle empty SDP Addr TODO about gosip HOT 1 OPEN

https://tools.ietf.org/html/rfc4566#section-5.7

A session description MUST contain either at least one "c=" field in
each media description or a single "c=" field at the session level.

So it's possible that we don't have a "c=" line in the upper SDP part but something like this:

v=0
o=- 3366701332 3366701332 IN IP4 1.2.3.4
s=-
t=0 0
m=audio 9 UDP/TLS/RTP/SAVPF 111 103 104 9 0 8 106 105 13 110 112 113 126
c=IN IP4 0.0.0.0
m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 102 123 127 122 125 107 108 109 124
c=IN IP4 0.0.0.0

Nevertheless, as you said a empty "c" line addr shouldn't happen and at least in the WebRTC space I think most ppl would just set it to IP6 :: (former it was IP4 0.0.0.0)

https://tools.ietf.org/html/draft-ietf-mmusic-trickle-ice-02#section-5.1

It is worth noting that the use of IP6 :: has been selected over IP4
0.0.0.0, even though [RFC3264] already gives the latter semantics
appropriate for such use. The reason for this choice is the historic
use of 0.0.0.0 as a means of putting a stream on hold [RFC2543] and
the ambiguity that this may cause with legacy libraries and
applications.

from gosip.