@Override
  protected MixedFormatSplit toSplitType(String splitId, MixedFormatSplitState splitState) {
    return splitState.toSourceSplit();
  }
SourceEvents may be received before this#pollNext.
amoro-format-mixed/amoro-mixed-flink/amoro-mixed-flink-common/src/main/java/org/apache/amoro/flink/read/hybrid/reader/MixedFormatSourceReader.java