@Override
  public Map<String, IWritableMemChunk> getMemChunkMap() {
    if (memChunk.count() == 0) {
      return Collections.emptyMap();
    }
    return Collections.singletonMap("", memChunk);
  }
Check whether this MemChunkGroup contains a measurement. If a VECTOR_PLACEHOLDER passed from
outer, always return true because AlignedMemChunkGroup existing.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/memtable/AlignedWritableMemChunkGroup.java