audio codec: if received no data, pass it further

This is an exception to the normal behavior used by the passive audio
mixer participant, that sends empty packets to request receiving the data.

It doesn't affect the normal UG behavior, because the situation when
nonzero_channels=0 doesn't occur otherwise. Either we receive at least
some packet from a frame and if not, the decoding doesn't take place.

Author: MartinPulec

src/audio/codec.cpp

@@ -344,6 +344,13 @@ audio_frame2 audio_codec_decompress(struct audio_codec_state *s, audio_frame2 *f
                 }
         }
 
+        if (nonzero_channels == 0 &&
+            frame->get_data_len() == 0) { // produced by acap/passive
+                ret.init(frame->get_channel_count(), AC_PCM, frame->get_bps(),
+                         frame->get_sample_rate());
+                return ret;
+        }
+
         if (nonzero_channels != frame->get_channel_count()) {
                 log_msg(LOG_LEVEL_WARNING,
                         "[Audio decompress] %d empty channel(s) returned!\n",