Enable NACK for audio even if there are no send streams.

webrtc/media/engine/webrtcvoiceengine.cc

 /*
  *  Copyright (c) 2004 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 1653 matching lines @@
   }
 
   // Cache the codecs in order to configure the channel created later.
   for (const auto& ch : send_streams_) {
     if (!SetSendCodecs(ch.second->channel(), ch.second->rtp_parameters())) {
       return false;
     }
   }
 
   // Set nack status on receive channels.
-  if (!send_streams_.empty()) {
-    for (const auto& kv : recv_streams_) {
-      SetNack(kv.second->channel(), send_codec_spec_.nack_enabled);
-    }
+  for (const auto& kv : recv_streams_) {
+    SetNack(kv.second->channel(), send_codec_spec_.nack_enabled);
   }
 
   // Check if the transport cc feedback has changed on the preferred send codec,
   // and in that case reconfigure all receive streams.
   if (recv_transport_cc_enabled_ != send_codec_spec_.transport_cc_enabled) {
     LOG(LS_INFO) << "Recreate all the receive streams because the send "
                     "codec has changed.";
     recv_transport_cc_enabled_ = send_codec_spec_.transport_cc_enabled;
     for (auto& kv : recv_streams_) {
       kv.second->RecreateAudioReceiveStream(recv_transport_cc_enabled_);
@@ skipping 868 matching lines @@
     }
   } else {
     LOG(LS_INFO) << "Stopping playout for channel #" << channel;
     engine()->voe()->base()->StopPlayout(channel);
   }
   return true;
 }
 }  // namespace cricket
 
 #endif  // HAVE_WEBRTC_VOICE