Fix H264RtpDepacketizer to handle empty payload RTP packet

[sile]

This PR fixes an issue that H264RtpDepacketizer cannot handle empty payload RTP packets that could be sent by Chrome (please see obs-studio#10353 comment for the detailed motivation).

How to reproduce the issue

Please apply the following diff to examples/media-receiver/main.cpp file and build the command.

diff --git a/examples/media-receiver/main.cpp b/examples/media-receiver/main.cpp
index 81e0d1d0..b2fe13c7 100644
--- a/examples/media-receiver/main.cpp
+++ b/examples/media-receiver/main.cpp
@@ -40,8 +40,10 @@ int main() {
                        std::cout << "Gathering State: " << state << std::endl;
                        if (state == rtc::PeerConnection::GatheringState::Complete) {
                                auto description = pc->localDescription();
+                               auto sdp = std::string(description.value());
+                               sdp += "a=extmap:2 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time\r\n";
                                json message = {{"type", description->typeString()},
-                                               {"sdp", std::string(description.value())}};
+                                               {"sdp", sdp}};
                                std::cout << message << std::endl;
                        }
                });
@@ -54,13 +56,13 @@ int main() {

                rtc::Description::Video media("video", rtc::Description::Direction::RecvOnly);
                media.addH264Codec(96);
-               media.setBitrate(
-                   3000); // Request 3Mbps (Browsers do not encode more than 2.5MBps from a webcam)

                auto track = pc->addTrack(media);

+               auto video_depacketizer = std::make_shared<rtc::H264RtpDepacketizer>();
                auto session = std::make_shared<rtc::RtcpReceivingSession>();
-               track->setMediaHandler(session);
+               video_depacketizer->addToChain(session);
+               track->setMediaHandler(video_depacketizer);

                track->onMessage(
                    [session, sock, addr](rtc::binary message) {

Execute media-receiver command and communicate with Chrome.
When media-receiver starts receiving video packets, it could emit the following error message:

$ ./media-receiver
...
2024-03-27 14:46:16.535 ERROR [281897] [rtc::impl::DtlsTransport::doRecv@990] DTLS recv: Unknown H264 RTP Packetization
2024-03-27 14:46:16.537 INFO  [281897] [rtc::impl::DtlsTransport::doRecv@994] DTLS closed

[paullouisageneau]

Thanks for the fix!