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

For example, zero rtt may be reported to:
BitrateControllerImpl::OnReceivedRtcpReceiverReport:
- SendSideBandwidthEstimation::UpdateReceiverBlock
- SendSideBandwidthEstimation::UpdateUmaStats
BitrateAllocator::OnNetworkChanged:
- ProtectionBitrateCalculator::SetTargetRates

Re-add check that was removed in https://codereview.webrtc.org/2422063002.

BUG=webrtc:6692

Review-Url: https://codereview.webrtc.org/2552883010
Cr-Commit-Position: refs/heads/master@{#15486}

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;