Replace NULL with nullptr in all C++ files.

webrtc/modules/rtp_rtcp/source/rtp_sender_audio.cc

 /*
  *  Copyright (c) 2012 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 190 matching lines @@
                                       dtmf_duration_samples,
                                       !dtmf_event_first_packet_sent_)) {
           return false;
         }
         dtmf_event_first_packet_sent_ = true;
         return true;
       }
     }
     return true;
   }
-  if (payload_size == 0 || payload_data == NULL) {
+  if (payload_size == 0 || payload_data == nullptr) {
     if (frame_type == kEmptyFrame) {
       // we don't send empty audio RTP packets
       // no error since we use it to drive DTMF when we use VAD
       return true;
     }
     return false;
   }
 
   std::unique_ptr<RtpPacketToSend> packet = rtp_sender_->AllocatePacket();
   packet->SetMarker(MarkerBit(frame_type, payload_type));
@@ skipping 118 matching lines @@
         TRACE_DISABLED_BY_DEFAULT("webrtc_rtp"), "Audio::SendTelephoneEvent",
         "timestamp", packet->Timestamp(), "seqnum", packet->SequenceNumber());
     result = rtp_sender_->SendToNetwork(std::move(packet), kAllowRetransmission,
                                         RtpPacketSender::kHighPriority);
     send_count--;
   } while (send_count > 0 && result);
 
   return result;
 }
 }  // namespace webrtc