message: Handle null structure for invalid scalability mode case
  The scalability mode could be something invalid set by user, in this
  case, |num_spatial_layers| should not be updated.
  Cr-Commit-Position: refs/heads/main@{#35985}
diff: @@ -70,8 +70,13 @@ void FrameEncodeMetadataWriter::OnEncoderInit(const VideoCodec& codec) {
  codec_settings_.ScalabilityMode() != "") {
  std::unique_ptr<ScalableVideoController> structure =
  CreateScalabilityStructure(codec_settings_.ScalabilityMode());
  - RTC_DCHECK(structure);
  + if (structure) {
  num_spatial_layers = structure->StreamConfig().num_spatial_layers;
  + } else {
  + // |structure| maybe nullptr if the scalability mode is invalid.
  + RTC_LOG(LS_WARNING) << "Cannot create ScalabilityStructure, since the "
  + "scalability mode is invalid";
  + }
  }
  num_spatial_layers_ = std::max(num_spatial_layers, size_t{1});
  }
  