Allow ack with no following token for rtcp-fb

[ekr]

switch (rtcpfb->feedback_type) {
  case SDP_RTCP_FB_ACK:
    type = SdpRtcpFbAttributeList::kAck;
    switch (rtcpfb->param.ack) {
      // TODO: sipcc doesn't seem to support ack with no following token
      case SDP_RTCP_FB_ACK_RPSI:
        parameter = SdpRtcpFbAttributeList::rpsi;
        break;
      case SDP_RTCP_FB_ACK_APP:
        parameter = SdpRtcpFbAttributeList::app;
        break;
      default:
        // Unknown type, ignore.
        continue;
    }

[docfaraday]

This is not a regression (we've never supported this), so I'm not inclined to make this a blocker on shipping.