Do not update OnReceivedRtcpReceiverReport if report block list is empty (and rtt zero).

webrtc/modules/bitrate_controller/bitrate_controller_impl.cc

Index: webrtc/modules/bitrate_controller/bitrate_controller_impl.cc
diff --git a/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc b/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc
index be0e0f12f21b49fcb70e30f0a4d7eaa6d2a3bcc9..9be67ef28c970541bcb36a9f5b81c7a8e52d7c5f 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc
@@ -38,6 +38,9 @@ class BitrateControllerImpl::RtcpBandwidthObserverImpl
   void OnReceivedRtcpReceiverReport(const ReportBlockList& report_blocks,
                                     int64_t rtt,
                                     int64_t now_ms) override {
+    if (report_blocks.empty())
+      return;
+
     int fraction_lost_aggregate = 0;
     int total_number_of_packets = 0;