Reland "Ensure RTCRtpSenders are always created with one encoding"

This is a reland of commit b8023690d9f0e150cfe698cd68b477903ac66178

Original change's description:
> Ensure RTCRtpSenders are always created with one encoding
>
> It is possible to have AddTransceiver calls with an empty array
> of encodings or AddTrack calls. In both cases, before negotiation,
> the sender's encodings array would be empty and it was not possible
> to update any value.
>
> Now, a default entry should be created in those cases, allowing to
> update the configuration before negotiation.
>
> Bug: webrtc:10567
> Change-Id: I1271e2965e1a97c1e472451e0ab8867fc24f6c2b
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/290994
> Auto-Submit: Florent Castelli <orphis@webrtc.org>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Commit-Queue: Florent Castelli <orphis@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#39126}

Bug: webrtc:10567
Change-Id: I558a95f7b587302b5e95f6ec26d1eb1fedf3dbed
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/291240
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39150}

pc/peer_connection.cc

@@ -1096,6 +1096,11 @@ PeerConnection::AddTransceiver(
     }
   }
 
+  // If no encoding parameters were provided, a default entry is created.
+  if (parameters.encodings.empty()) {
+    parameters.encodings.push_back({});
+  }
+
   if (UnimplementedRtpParameterHasValue(parameters)) {
     LOG_AND_RETURN_ERROR(
         RTCErrorType::UNSUPPORTED_PARAMETER,

pc/peer_connection_integrationtest.cc

@@ -2834,6 +2834,14 @@ TEST_P(PeerConnectionIntegrationTest, UnsignaledSsrcGetParametersVideo) {
   EXPECT_TRUE(parameters.encodings[0].ssrc.has_value());
 }
 
+TEST_P(PeerConnectionIntegrationTest,
+       GetParametersHasEncodingsBeforeNegotiation) {
+  ASSERT_TRUE(CreatePeerConnectionWrappers());
+  auto sender = caller()->AddTrack(caller()->CreateLocalVideoTrack());
+  auto parameters = sender->GetParameters();
+  EXPECT_EQ(parameters.encodings.size(), 1u);
+}
+
 // Test that if a track is removed and added again with a different stream ID,
 // the new stream ID is successfully communicated in SDP and media continues to
 // flow end-to-end.
@@ -3410,6 +3418,28 @@ TEST_F(PeerConnectionIntegrationTestUnifiedPlan,
   }
 }
 
+TEST_F(PeerConnectionIntegrationTestUnifiedPlan,
+       GetParametersHasEncodingsBeforeNegotiation) {
+  ASSERT_TRUE(CreatePeerConnectionWrappers());
+  auto result = caller()->pc()->AddTransceiver(cricket::MEDIA_TYPE_VIDEO);
+  auto transceiver = result.MoveValue();
+  auto parameters = transceiver->sender()->GetParameters();
+  EXPECT_EQ(parameters.encodings.size(), 1u);
+}
+
+TEST_F(PeerConnectionIntegrationTestUnifiedPlan,
+       GetParametersHasInitEncodingsBeforeNegotiation) {
+  ASSERT_TRUE(CreatePeerConnectionWrappers());
+  RtpTransceiverInit init;
+  init.send_encodings.push_back({});
+  init.send_encodings[0].max_bitrate_bps = 12345;
+  auto result = caller()->pc()->AddTransceiver(cricket::MEDIA_TYPE_VIDEO, init);
+  auto transceiver = result.MoveValue();
+  auto parameters = transceiver->sender()->GetParameters();
+  ASSERT_EQ(parameters.encodings.size(), 1u);
+  EXPECT_EQ(parameters.encodings[0].max_bitrate_bps, 12345);
+}
+
 INSTANTIATE_TEST_SUITE_P(
     PeerConnectionIntegrationTest,
     PeerConnectionIntegrationTest,

pc/rtp_transmission_manager.cc

@@ -150,10 +150,11 @@ RtpTransmissionManager::AddTrackPlanB(
       (track->kind() == MediaStreamTrackInterface::kAudioKind
            ? cricket::MEDIA_TYPE_AUDIO
            : cricket::MEDIA_TYPE_VIDEO);
-  auto new_sender =
+  auto new_sender = CreateSender(
-      CreateSender(media_type, track->id(), track, adjusted_stream_ids,
+      media_type, track->id(), track, adjusted_stream_ids,
-                   init_send_encodings ? *init_send_encodings
+      init_send_encodings
-                                       : std::vector<RtpEncodingParameters>());
+          ? *init_send_encodings
+          : std::vector<RtpEncodingParameters>(1, RtpEncodingParameters{}));
   if (track->kind() == MediaStreamTrackInterface::kAudioKind) {
     new_sender->internal()->SetMediaChannel(voice_media_send_channel());
     GetAudioTransceiver()->internal()->AddSender(new_sender);
@@ -217,10 +218,11 @@ RtpTransmissionManager::AddTrackUnifiedPlan(
     if (FindSenderById(sender_id)) {
       sender_id = rtc::CreateRandomUuid();
     }
-    auto sender = CreateSender(media_type, sender_id, track, stream_ids,
+    auto sender = CreateSender(
+        media_type, sender_id, track, stream_ids,
         init_send_encodings
             ? *init_send_encodings
-                                   : std::vector<RtpEncodingParameters>());
+            : std::vector<RtpEncodingParameters>(1, RtpEncodingParameters{}));
     auto receiver = CreateReceiver(media_type, rtc::CreateRandomUuid());
     transceiver = CreateAndAddTransceiver(sender, receiver);
     transceiver->internal()->set_created_by_addtrack(true);
@@ -411,7 +413,7 @@ void RtpTransmissionManager::AddAudioTrack(AudioTrackInterface* track,
   // Normal case; we've never seen this track before.
   auto new_sender = CreateSender(cricket::MEDIA_TYPE_AUDIO, track->id(),
                                  rtc::scoped_refptr<AudioTrackInterface>(track),
-                                 {stream->id()}, {});
+                                 {stream->id()}, {{}});
   new_sender->internal()->SetMediaChannel(voice_media_send_channel());
   GetAudioTransceiver()->internal()->AddSender(new_sender);
   // If the sender has already been configured in SDP, we call SetSsrc,
@@ -458,7 +460,7 @@ void RtpTransmissionManager::AddVideoTrack(VideoTrackInterface* track,
   // Normal case; we've never seen this track before.
   auto new_sender = CreateSender(cricket::MEDIA_TYPE_VIDEO, track->id(),
                                  rtc::scoped_refptr<VideoTrackInterface>(track),
-                                 {stream->id()}, {});
+                                 {stream->id()}, {{}});
   new_sender->internal()->SetMediaChannel(video_media_send_channel());
   GetVideoTransceiver()->internal()->AddSender(new_sender);
   const RtpSenderInfo* sender_info =