code: def start_index(self) -> Union[int, None]:
      if self._data.empty:
          return None
      return self._data.index[0]
  
imports: from typing import Union, List, Dict, Tuple
  import unittest
  import pandas
  import numpy
  import io
  from .data import GetData
  from ..None import init
  from ..constant import REG_CN, REG_TW
  from qlib.data.filter import NameDFilter
  from qlib.data import D
  from qlib.data.data import Cal, DatasetD
  from qlib.data.storage import CalendarStorage, InstrumentStorage, FeatureStorage, CalVT, InstKT, InstVT
label_desc: Use image node_id 3 for calling the MockFeatureStorage obj's underlying member method code with example usage: obj.start_index() and returns: self, None