Make stats analysis working with empty layers (bitrate=0)

Setting layer's target bitrate to zero is a valid scanario. Example of command line: out/debug/video_codec_perf_tests --gtest_also_run_disabled_tests --gtest_filter=*EncodeDecode --encoder=libvpx-vp9 --decoder=libvpx-vp9 --scalability_mode=L2T1_KEY --width=640 --height=360 --bitrate_kbps=0,400

Bug: webrtc:14852
Change-Id: If0619dde1da254699939d56f2971dd0bc8391d95
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/349160
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42174}
diff --git a/test/video_codec_tester.cc b/test/video_codec_tester.cc
index ac671a9..4706eb6 100644
--- a/test/video_codec_tester.cc
+++ b/test/video_codec_tester.cc
@@ -574,6 +574,10 @@
     }
 
     int num_encoded_frames = stream.frame_size_bytes.NumSamples();
+    if (num_encoded_frames == 0) {
+      return stream;
+    }
+
     const Frame& first_frame = frames.front();
 
     Filter filter_all_layers{.min_timestamp_rtp = filter.min_timestamp_rtp,