Conference bridge should transmit silence frame when level is zero

[bennylp]

Currently the conference bridge will transmit PJMEDIA_FRAME_TYPE_NONE to a destination slot when the level is zero, regardless whether there is a source port transmitting to that particular destination slot:

  if (cport->tx_setting==PJMEDIA_PORT_MUTE || cport->srv_level==0) {
     frame.type = PJMEDIA_FRAME_TYPE_NONE;
     pjmedia_port_put_frame(cport->port, &frame);
  }

This will cause WAV writer to write nothing to the file, thus causing silence gap to not be written.

The solution, the bridge should distinguish between no listener situation and silence situation:

    if (cport->tx_setting == PJMEDIA_PORT_MUTE || cport->transmitter_cnt==0) {
       // Transmit TYPE_NONE frame
       ...
    } else if (cport->src_level==0) {
       // Transmit silence PCM frame
       ...
    }