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

webrtc/media/engine/webrtcvoiceengine.cc

Index: webrtc/media/engine/webrtcvoiceengine.cc
diff --git a/webrtc/media/engine/webrtcvoiceengine.cc b/webrtc/media/engine/webrtcvoiceengine.cc
index ed67ce84ba00cc520872b1a17e0ea8bdbb579160..476b21ae78847a38efe24d7338ce7aec7b65af09 100644
--- a/webrtc/media/engine/webrtcvoiceengine.cc
+++ b/webrtc/media/engine/webrtcvoiceengine.cc
@@ -1671,10 +1671,8 @@ bool WebRtcVoiceMediaChannel::SetSendCodecs(
   }
 
   // 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,