Allow parsing empty RTCP TargetBitrate messages, but stop sending them.

webrtc/modules/rtp_rtcp/source/rtcp_packet/extended_reports.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 186 matching lines @@
     LOG(LS_WARNING)
         << "Two Voip Metric blocks found in same Extended Report packet";
     return;
   }
   voip_metric_block_.emplace();
   voip_metric_block_->Parse(block);
 }
 
 void ExtendedReports::ParseTargetBitrateBlock(const uint8_t* block,
                                               uint16_t block_length) {
-  target_bitrate_ = rtc::Optional<TargetBitrate>(TargetBitrate());
-  if (!target_bitrate_->Parse(block, block_length))
-    target_bitrate_ = rtc::Optional<TargetBitrate>();
+  target_bitrate_.emplace();
+  target_bitrate_->Parse(block, block_length);
 }
 }  // namespace rtcp
 }  // namespace webrtc